libc: Build all i386 sources for amd64 lib32

Having the symbols exported by libc differ between i386 and amd64 lib32
is questionable. Since these files build just fine today, stop guarding
them with !defined(COMPAT_32BIT). Whether or not they work at run time
is a different matter, but an i386 jail would be similarly affected if
not, so that's not a problem with lib32.

Reviewed by: kib, jhb, imp
Differential Revision: https://reviews.freebsd.org/D40937