runtime: move timeUnit to a single place

The timeUnit is now the same type everywhere. Move it to a single place
and add some documentation to it.

src/runtime/runtime_unix.go

@@ -61,8 +61,6 @@ func clock_gettime(clk_id int32, ts *timespec) {
 	}
 }
 
-type timeUnit int64
-
 // Note: tv_sec and tv_nsec normally vary in size by platform. However, we're
 // using the time64 variant (see clock_gettime above), so the formats are the
 // same between 32-bit and 64-bit architectures.