The memory sanitizer (msan) is a nice compiler feature that can
dynamically check for memory errors in C code.  It's not useful for Go
code, since Go is memory safe.  But it is useful to be able to use the
memory sanitizer on C code that is linked into a Go program via cgo.
Without this change it does not work, as msan considers memory passed
from Go to C as uninitialized.

To make this work, change the runtime to call the C mmap function when
using cgo.  When using msan the mmap call will be intercepted and marked
as returning initialized memory.

Work around what appears to be an msan bug by calling malloc before we
call mmap.

Change-Id: I8ab7286d7595ae84782f68a98bef6d3688b946f9
Reviewed-on: https://go-review.googlesource.com/15170
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>