linux: Add time64 semtimedop support

Either the __NR_semtimedop_time64 (for 32-bit) or the __NR_semtimedop
(for 64-bit) syscall 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_ipc or __NR_semtimeop (for 32-bit time_t).

Checked on x86_64-linux-gnu and i686-linux-gnu (on 5.4 and on 4.15
kernel).

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