libn64/os: Add a libn64_time() syscall.

libn64/os: Add a libn64_time() syscall.

The syscall returns a timespec struct that has two fields
populated: tv_sec and tv_usec. Since the N64 doesn't have a
RTC, this function does the "next best thing" and returns
the time offset since libn64 started.

The syscall uses the COUNT register to precisely measure
time. It can effectively count upto a couple hundred years
before it "rolls over".

Since the COUNT register is effectively driven at "only",
46.875MHz the time source is just barely accurate within a
hundred usec or so.

Signed-off-by: Tyler J. Stachecki <stachecki.tyler@gmail.com>