runtime: call mmap with MAP_FIXED on BSDs in race mode

This change makes it so that reserving more of the address space for the
heap calls mmap with MAP_FIXED in race mode. Race mode requires certain
guarantees on where the heap is located in the address space, and on
Darwin 10.10 it appears that the kernel may end up ignoring the hint
quite often (#26475). Using MAP_FIXED is relatively OK in race mode
because nothing else should be mapped in the memory region provided by
the initial hints.

Fixes #26475.

Change-Id: Id7ac1534ee74f6de491bc04441f27dbda09f0285
Reviewed-on: https://go-review.googlesource.com/c/153897


Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>