bswap: Rewrite cpu_to_<endian><type>u with {ld,st}<type>_<endian>_p

We've now optimized the ld/st versions; reuse that for the "legacy"
versions.  Always use inlines so that we get the type checking that
we expect.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
