Add __builtin_bswap32/64 builtins for GCC 4.7

The builtins added by commit 7409c592 (Add llvm-gcc/g++ builtins,
2010-09-07) are now available in GCC 4.7, so remove the __llvm__
conditional in "GCC_XML/Support/GCC/4.7/gccxml_builtins.h".

Reported-by: Matěj Cepl <mcepl@redhat.com>