Add __builtin_bswap32/64 builtins for all GCC >= 4.2 (#13430)

The builtins added by commit 7409c592 (Add llvm-gcc/g++ builtins,
2010-09-07) are now available in glibc 2.16 so remove the __llvm__
conditional in "GCC_XML/Support/GCC/*/gccxml_builtins.h".  This
generalizes the parent commit.