8.10.3 fails to build on riscv64

Commit 2e3a9399 introduced atomic operations on 16-bit objects. On riscv64, sub-word atomic operations requires linking against libatomic. This causes a link failure while building the stage2 compiler:

"inplace/bin/ghc-stage1" -hisuf hi -osuf  o -hcsuf hc -static  -O -H64m -fllvm -Wall  -Iincludes -Iincludes/dist -Iincludes/dist-derivedconstants/header -Iincludes/dist-ghcconstants/header   -this-unit-id ghc-8.10.3 -hide-all-packages -i -icompiler/. -icompiler/backpack -icompiler/basicTypes -icompiler/cmm -icompiler/coreSyn -icompiler/deSugar -icompiler/ghci -icompiler/iface -icompiler/llvmGen -icompiler/main -icompiler/nativeGen -icompiler/parser -icompiler/prelude -icompiler/profiling -icompiler/rename -icompiler/simplCore -icompiler/simplStg -icompiler/specialise -icompiler/stgSyn -icompiler/stranal -icompiler/typecheck -icompiler/types -icompiler/utils -icompiler/hieFile -icompiler/stage2/build -Icompiler/stage2/build -icompiler/stage2/build/./autogen -Icompiler/stage2/build/./autogen -Icompiler/. -Icompiler/parser -Icompiler/utils -Icompiler/../rts/dist/build -Icompiler/stage2 -Icompiler/stage2/build/. -Icompiler/stage2/build/parser -Icompiler/stage2/build/utils -Icompiler/stage2/build/../rts/dist/build -Icompiler/stage2/build/stage2   -optP-DHAVE_INTERNAL_INTERPRETER -optP-DINTEGER_GMP -optP-DCAN_LOAD_DLL -optP-Iincludes/dist-install/build -optP-include -optPcompiler/stage2/build/./autogen/cabal_macros.h -package-id array-0.5.4.0 -package-id base-4.14.1.0 -package-id binary-0.8.8.0 -package-id bytestring-0.10.12.0 -package-id containers-0.6.2.1 -package-id deepseq-1.4.4.0 -package-id directory-1.3.6.0 -package-id filepath-1.4.2.1 -package-id ghc-boot-8.10.3 -package-id ghc-boot-th-8.10.3 -package-id ghc-heap-8.10.3 -package-id ghci-8.10.3 -package-id hpc-0.6.1.0 -package-id integer-gmp-1.0.3.0 -package-id process-1.6.9.0 -package-id template-haskell-2.16.0.0 -package-id terminfo-0.4.1.4 -package-id time-1.9.3 -package-id transformers-0.5.6.2 -package-id unix-2.7.2.2 -Wall -Wno-name-shadowing -Wnoncanonical-monad-instances -Wnoncanonical-monoid-instances -this-unit-id ghc -XHaskell2010 -XNoImplicitPrelude -optc-DTHREADED_RTS -Iincludes/dist-install/build -Rghc-timing -O2 -Wcpp-undef  -no-user-package-db -rtsopts      -Wnoncanonical-monad-instances  -outputdir compiler/stage2/build  -dynamic-too -c compiler/iface/IfaceType.hs -o compiler/stage2/build/IfaceType.o -dyno compiler/stage2/build/IfaceType.dyn_o
/usr/bin/ld.bfd: /home/abuild/rpmbuild/BUILD/ghc-8.10.3/rts/dist/build/libHSrts_thr_p.a(Evac.thr_p_o): undefined reference to symbol '__atomic_fetch_or_2@@LIBATOMIC_1.0'
/usr/bin/ld.bfd: /usr/lib64/libatomic.so.1: error adding symbols: DSO missing from command line