This currently generates many linker warnings of this form, due to
defining mem(cpy|move|set) in every object file:
```
ld: warning: '.../build/projects/compiler-rt/lib/interception/CMakeFiles/RTInterception.ios.dir/interception_linux.cpp.o' has malformed LC_DYSYMTAB, expected 6 undefined symbols to start at index 1, found 3 undefined symbols starting at index 1
```
In order for this to actually replace these symbols on mach-o, they
would need a leading underscore, e.g. `.set _memcpy,
___sanitizer_internal_memcpy`. However doing so does not fix the
warnings, and furthermore it ends up replacing `REAL(memcpy)` calls with
`__sanitizer_internal_memcpy` in places such as
`__asan::Allocator::Reallocate`. There is no way on Apple platforms to
recreate the intended behaviour, so let's just disable this on them to
reduce warning noise.
rdar://123771479
cfdbbb8fcc