[llvm] [LTO] Allow target-specific module splittting (PR #83128)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

@Pierre-vh This patch breaks our downstream build (with GCC 10.2.1):

```
FAILED: bin/llvm-split 
: && /usr/bin/c++ -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -fno-lifetime-dse -w -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG -Wl,-rpath-link,/home/dtcxzyw/llvm-build/./lib  -Wl,--gc-sections tools/llvm-split/CMakeFiles/llvm-split.dir/llvm-split.cpp.o -o bin/llvm-split  -Wl,-rpath,"\$ORIGIN/../lib:/home/dtcxzyw/llvm-build/lib:"  lib/libLLVMX86AsmParser.so.19.0git  lib/libLLVMRISCVAsmParser.so.19.0git  lib/libLLVMX86CodeGen.so.19.0git  lib/libLLVMRISCVCodeGen.so.19.0git  lib/libLLVMX86Desc.so.19.0git  lib/libLLVMRISCVDesc.so.19.0git  lib/libLLVMX86Info.so.19.0git  lib/libLLVMRISCVInfo.so.19.0git  -lpthread  lib/libLLVMCodeGen.so.19.0git  lib/libLLVMTarget.so.19.0git  lib/libLLVMBitWriter.so.19.0git  lib/libLLVMTransformUtils.so.19.0git  lib/libLLVMIRReader.so.19.0git  lib/libLLVMMC.so.19.0git  lib/libLLVMCore.so.19.0git  lib/libLLVMSupport.so.19.0git  -Wl,-rpath-link,/home/dtcxzyw/llvm-build/lib && :
/usr/bin/ld: tools/llvm-split/CMakeFiles/llvm-split.dir/llvm-split.cpp.o: undefined reference to symbol '_ZN4llvm6TripleC1ERKNS_5TwineE'
/usr/bin/ld: /home/dtcxzyw/llvm-build/./lib/libLLVMTargetParser.so.19.0git: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
```

https://github.com/llvm/llvm-project/pull/83128