[libc++] Remove definition of _LIBCPP_ALIGNOF for GCC in C++03 mode

That definition is known to be potentially incorrect, and we don't support
GCC in C++03 mode anyway.

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 4f57a126c4e763e3041c04f0b22e91200506dcc6
1 file changed