This seems intentional but on the other hand the error message says "Please report this as a GHC bug"

"inplace/bin/ghc-stage1" -static  -H32m -O -lffi -optl-pthread -optl-B/usr/bin/ld.gold -Wall   -Iincludes -Iincludes/dist -Iincludes/dist-derivedconstants/header -Iincludes/dist-ghcconstants/header -Irts -Irts/dist/build -DCOMPILING_RTS -this-unit-id rts -optc-DNOSMP -dcmm-lint      -i -irts -irts/dist/build -Irts/dist/build -irts/dist/build/./autogen -Irts/dist/build/./autogen            -O2 -Wcpp-undef    -Wnoncanonical-monad-instances  -c rts/StgStartup.cmm -o rts/dist/build/StgStartup.o
"inplace/bin/ghc-stage1" -static  -H32m -O -lffi -optl-pthread -optl-B/usr/bin/ld.gold -Wall   -Iincludes -Iincludes/dist -Iincludes/dist-derivedconstants/header -Iincludes/dist-ghcconstants/header -Irts -Irts/dist/build -DCOMPILING_RTS -this-unit-id rts -optc-DNOSMP -dcmm-lint      -i -irts -irts/dist/build -Irts/dist/build -irts/dist/build/./autogen -Irts/dist/build/./autogen            -O2 -Wcpp-undef    -Wnoncanonical-monad-instances  -c rts/PrimOps.cmm -o rts/dist/build/PrimOps.o
"inplace/bin/ghc-stage1" -static  -H32m -O -lffi -optl-pthread -optl-B/usr/bin/ld.gold -Wall   -Iincludes -Iincludes/dist -Iincludes/dist-derivedconstants/header -Iincludes/dist-ghcconstants/header -Irts -Irts/dist/build -DCOMPILING_RTS -this-unit-id rts -optc-DNOSMP -dcmm-lint      -i -irts -irts/dist/build -Irts/dist/build -irts/dist/build/./autogen -Irts/dist/build/./autogen            -O2 -Wcpp-undef    -Wnoncanonical-monad-instances  -c rts/Updates.cmm -o rts/dist/build/Updates.o
ghc-stage1: panic! (the 'impossible' happened)
  (GHC version 8.4.3 for arm-unknown-linux):
	Failed to lookup the datalayout for armv7l-unknown-linux-gnueabi; available targets: ["i386-unknown-windows","i686-unknown-windows","x86_64-unknown-windows","arm-unknown-linux-gnueabihf","armv6-unknown-linux-gnueabihf","armv6l-unknown-linux-gnueabihf","armv7-unknown-linux-gnueabihf","armv7a-unknown-linux-gnueabi","armv7l-unknown-linux-gnueabihf","aarch64-unknown-linux-gnu","aarch64-unknown-linux","i386-unknown-linux-gnu","i386-unknown-linux","x86_64-unknown-linux-gnu","x86_64-unknown-linux","armv7-unknown-linux-androideabi","aarch64-unknown-linux-android","arm-unknown-nto-qnx-eabi","i386-apple-darwin","x86_64-apple-darwin","armv7-apple-ios","aarch64-apple-ios","i386-apple-ios","x86_64-apple-ios"]
CallStack (from HasCallStack):
  error, called at compiler/llvmGen/LlvmCodeGen.hs:96:24 in ghc:LlvmCodeGen

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

rts/ghc.mk:295: recipe for target 'rts/dist/build/StgStartup.o' failed
make[3]: *** [rts/dist/build/StgStartup.o] Error 1
make[3]: *** Waiting for unfinished jobs....
ghc-stage1: panic! (the 'impossible' happened)
  (GHC version 8.4.3 for arm-unknown-linux):
	Failed to lookup the datalayout for armv7l-unknown-linux-gnueabi; available targets: ["i386-unknown-windows","i686-unknown-windows","x86_64-unknown-windows","arm-unknown-linux-gnueabihf","armv6-unknown-linux-gnueabihf","armv6l-unknown-linux-gnueabihf","armv7-unknown-linux-gnueabihf","armv7a-unknown-linux-gnueabi","armv7l-unknown-linux-gnueabihf","aarch64-unknown-linux-gnu","aarch64-unknown-linux","i386-unknown-linux-gnu","i386-unknown-linux","x86_64-unknown-linux-gnu","x86_64-unknown-linux","armv7-unknown-linux-androideabi","aarch64-unknown-linux-android","arm-unknown-nto-qnx-eabi","i386-apple-darwin","x86_64-apple-darwin","armv7-apple-ios","aarch64-apple-ios","i386-apple-ios","x86_64-apple-ios"]
CallStack (from HasCallStack):
  error, called at compiler/llvmGen/LlvmCodeGen.hs:96:24 in ghc:LlvmCodeGen

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

rts/ghc.mk:295: recipe for target 'rts/dist/build/Updates.o' failed
make[3]: *** [rts/dist/build/Updates.o] Error 1
ghc-stage1: panic! (the 'impossible' happened)
  (GHC version 8.4.3 for arm-unknown-linux):
	Failed to lookup the datalayout for armv7l-unknown-linux-gnueabi; available targets: ["i386-unknown-windows","i686-unknown-windows","x86_64-unknown-windows","arm-unknown-linux-gnueabihf","armv6-unknown-linux-gnueabihf","armv6l-unknown-linux-gnueabihf","armv7-unknown-linux-gnueabihf","armv7a-unknown-linux-gnueabi","armv7l-unknown-linux-gnueabihf","aarch64-unknown-linux-gnu","aarch64-unknown-linux","i386-unknown-linux-gnu","i386-unknown-linux","x86_64-unknown-linux-gnu","x86_64-unknown-linux","armv7-unknown-linux-androideabi","aarch64-unknown-linux-android","arm-unknown-nto-qnx-eabi","i386-apple-darwin","x86_64-apple-darwin","armv7-apple-ios","aarch64-apple-ios","i386-apple-ios","x86_64-apple-ios"]
CallStack (from HasCallStack):
  error, called at compiler/llvmGen/LlvmCodeGen.hs:96:24 in ghc:LlvmCodeGen

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

rts/ghc.mk:295: recipe for target 'rts/dist/build/PrimOps.o' failed

https://buildd.debian.org/status/fetch.php?pkg=ghc&arch=armel&ver=8.4.3-1&stamp=1527769922&raw=0