Don't link plugins with target code

I'm working on #14335: supporting plugins with -fexternal-interpreter by having them use their own environment which uses the internal interpreter. However it turns out that GHC links plugins with target code at least in some cases, which makes the linking phase fails.

With the most simple example -- a plugin that does nothing used to compile a program that does nothing -- we get the following linker invocation:

cc '-fuse-ld=lld' -Wl,--no-as-needed -o DumbProgram -no-pie -fPIC -U__PIC__ -D__PIC__ -Wl,--gc-sections TXXXXX.o '-L/tmp/ghctest-tqzmgxss/test   spaces/testsuite/tests/plugins/TXXXXX.run/simple-plugin/pkg.TXXXXX/install/lib/x86_64-linux-ghc-9.3.20210804' -Xlinker -rpath-link -Xlinker '/tmp/ghctest-tqzmgxss/test   spaces/testsuite/tests/plugins/TXXXXX.run/simple-plugin/pkg.TXXXXX/install/lib/x86_64-linux-ghc-9.3.20210804' -Xlinker -rpath -Xlinker '/tmp/ghctest-tqzmgxss/test   spaces/testsuite/tests/plugins/TXXXXX.run/simple-plugin/pkg.TXXXXX/install/lib/x86_64-linux-ghc-9.3.20210804' -L/home/hsyl20/projects/ghc/master/_build/stage1/lib/../lib/x86_64-linux-ghc-9.3.20210804 -Xlinker -rpath-link -Xlinker /home/hsyl20/projects/ghc/master/_build/stage1/lib/../lib/x86_64-linux-ghc-9.3.20210804 -Xlinker -rpath -Xlinker /home/hsyl20/projects/ghc/master/_build/stage1/lib/../lib/x86_64-linux-ghc-9.3.20210804 /tmp/ghc126596_0/ghc_4.o /tmp/ghc126596_0/ghc_7.o -Wl,-u,hs_atomic_add64 -Wl,-u{...many others...} -lHSsimple-plugin-0.1-95cpf37BE66F7W09my2aO-ghc9.3.20210804 -lHSghc-9.3-ghc9.3.20210804 -lHSterminfo-0.4.1.4-ghc9.3.20210804 -lHSprocess-1.6.13.1-ghc9.3.20210804 -lHSparsec-3.1.14.0-ghc9.3.20210804 -lHStext-1.2.4.2-ghc9.3.20210804 -lHShpc-0.6.1.0-ghc9.3.20210804 -lHSghci-9.3-ghc9.3.20210804 -lHSghc-heap-9.3-ghc9.3.20210804 -lHSghc-boot-9.3-ghc9.3.20210804 -lHSexceptions-0.10.4-ghc9.3.20210804 -lHStemplate-haskell-2.18.0.0-ghc9.3.20210804 -lHSpretty-1.1.3.6-ghc9.3.20210804 -lHSghc-boot-th-9.3-ghc9.3.20210804 -lHSstm-2.5.0.0-ghc9.3.20210804 -lHSmtl-2.2.2-ghc9.3.20210804 -lHStransformers-0.5.6.2-ghc9.3.20210804 -lHSdirectory-1.3.6.1-ghc9.3.20210804 -lHSunix-2.7.2.2-ghc9.3.20210804 -lHStime-1.11.1.1-ghc9.3.20210804 -lHSfilepath-1.4.2.1-ghc9.3.20210804 -lHSbinary-0.8.8.0-ghc9.3.20210804 -lHScontainers-0.6.4.1-ghc9.3.20210804 -lHSbytestring-0.11.1.0-ghc9.3.20210804 -lHSdeepseq-1.4.4.0-ghc9.3.20210804 -lHSarray-0.5.4.0-ghc9.3.20210804 -lHSbase-4.16.0.0-ghc9.3.20210804 -lHSghc-bignum-1.2-ghc9.3.20210804 -lHSghc-prim-0.8.0-ghc9.3.20210804 -lHSrts-1.0.2-ghc9.3.20210804 -lffi -ltinfo -lrt -lutil -ldl -lpthread -lgmp -lc -lm -lm -lrt -ldl -lpthread -lnuma

Notice -lHSsimple-plugin-0.1-95cpf37BE66F7W09my2aO-ghc9.3.20210804, -lHSghc-9.3-ghc9.3.20210804 and similar flags for every GHC dependency.