[compiler-rt][Darwin] Fix linker errors for check-asan
A recent change broke `ninja check-asan` on Darwin by causing an error
during linking of ASan unit tests [1].
Move the addition of `-ObjC` compiler flag outside of the new
`if(COMPILER_RT_STANDALONE_BUILD)` block. It doesn't add any global
flags (e.g, `${CMAKE_CXX_FLAGS}`) and the decision to add is based
solely on source paths (`${source_rpath}`).
[1]
8b2fcc42b895, https://reviews.llvm.org/D84466
Differential Revision: https://reviews.llvm.org/D85057