Re-apply "[CMake][compiler-rt][AArch64] Avoid preprocessing LSE builtins separately"

aa772fc8 (D92530) has landed fixing relocations on Darwin.
3000c19d (D93236) has landed working around an assembly parser bug on Darwin.
Previous quick-fix d9697c2e (D93198) included in this commit.

Invoking the preprocessor ourselves is fragile and would require us to replicate CMake's handling of definitions, compiler flags, etc for proper compatibility.
In my toolchain builds this notably resulted in a bunch of warnings from unused flags as my CMAKE_C_FLAGS includes CPU-specific optimization options.
Notably this part was already duplicating the logic for VISIBILITY_HIDDEN define.

Instead, symlink the files and set the proper set of defines on each.
This should also be faster as we avoid invoking the compiler multiple times.

Fixes https://llvm.org/PR48494

Differential Revision: https://reviews.llvm.org/D93278