linux: Add time64 sigtimedwait support

The syscall __NR_sigtimedwait_time64 (for 32-bit) or __NR_sigtimedwait
(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_rt_sigtimedwait (32-bit time_t).

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

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