swab.c(libc): use a simplified version of byte swapping

This version of swab function simplifies the logic of swapping adjacent
bytes. Previous version of swab() used an arbitrary unrolling, which was
relevant back in the day but unnecessary for modern compilers, as if the
input size is known at compile time, they can do it automatically.

This version of swab() is inspired by musl.
A similar version can be found at: https://github.com/openbsd/src/blob/master/lib/libc/string/swab.c

Signed-off-by: rilysh <nightquick@proton.me>
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1086