[libc] Add byteswap.h header with bswap_16/bswap_32/bswap_64 (#196278)

Added the Linux byteswap.h header providing bswap_16, bswap_32, and
bswap_64 as macros expanding to __builtin_bswap{16,32,64}. These are
always inlined by the compiler to single instructions.

Follows the existing endian.h / endian-macros.h pattern.

Assisted-by: Automated tooling, human reviewed.
GitOrigin-RevId: 7554007c806e5fdcfd4b034c2a462089428890da