x86/x32: use time64 versions of sigtimedwait and recvmmsg

x32 has always followed the time64 calling conventions of these
syscalls, which required a special hack in compat_get_timespec
aka get_old_timespec32 to continue working.

Since we now have the time64 syscalls, use those explicitly.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>