m68k: Enforce 4-byte alignment on internal locks (BZ #29537)

A new internal definition, __LIBC_LOCK_ALIGNMENT, is used to force
the 4-byte alignment only for m68k, other architecture keep the
natural alignment of the type used internally (and hppa does not
require 16-byte alignment for kernel-assisted CAS).

Reviewed-by: Florian Weimer <fweimer@redhat.com>
(cherry picked from commit aeb4d2e9)