runtime: use _libgo_off_t_type when calling C mmap
runtime: use _libgo_off_t_type when calling C mmap

The last argument to the C mmap function is type off_t, not uintptr.
On some 32-bit systems, off_t is larger than uintptr.

Based on patch by Sören Tempel.

Change-Id: Ib6f9d5bcd4734f99a72889e2282b97ec3c8fbd34
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/445735
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend