[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] 8b2fcc42, https://reviews.llvm.org/D84466

Differential Revision: https://reviews.llvm.org/D85057