linux: Add time64 recvmmsg support

The wire-up syscall __NR_recvmmsg_time64 (for 32-bit) or
__NR_recvmmsg (for 64-bit) is used as default.  The 32-bit fallback
is used iff __ASSUME_TIME64_SYSCALLS is not defined, which assumes the
kernel ABI provides either __NR_socketcall or __NR_recvmmsg
(32-bit time_t).

It does not handle the timestamps on ancillary data (SCM_TIMESTAMPING
records).

Checked on x86_64-linux-gnu and i686-linux-gnu.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>