[libc++] Only check for GCC's empty string storage on macOS and iOS

We don't need to do that on other Apple platforms, since they never
shipped libstdc++. I also added a comment extracted from the original
commit by Howard Hinnant (e115af2777f6).

Differential Revision: https://reviews.llvm.org/D91359

GitOrigin-RevId: 2728293bbc345a7c80fcd5ee0cc2ac3eac582bc8
1 file changed