byteswap.h: fix gcc ver test for __builtin_bswap{32,64}

byteswap.h: fix gcc ver test for __builtin_bswap{32,64}

The __builtin_bswap* functions were introduced in gcc-4.3, not gcc-4.2.
Fix the __GNUC_PREREQ tests to reflect this.

Otherwise trying to compile code with gcc-4.2 falls down:
In file included from /usr/include/endian.h:60,
                 from /usr/include/ctype.h:40,
/usr/include/bits/byteswap.h: In function 'unsigned int __bswap_32(unsigned int)':
/usr/include/bits/byteswap.h:46: error: '__builtin_bswap32' was not declared in this scope
/usr/include/bits/byteswap.h: In function 'long long unsigned int __bswap_64(long long unsigned int)':
/usr/include/bits/byteswap.h:110: error: '__builtin_bswap64' was not declared in this scope

Signed-off-by: Mike Frysinger <vapier@gentoo.org>