The -msan option causes the compiler to add instrumentation for the
C/C++ memory sanitizer.  Every memory read/write will be preceded by
a call to msanread/msanwrite.

This CL passes tests but is not usable by itself.  The actual
implementation of msanread/msanwrite in the runtime package, and support
for -msan in the go tool and the linker, and tests, will follow in
subsequent CLs.

Change-Id: I3d517fb3e6e65d9bf9433db070a420fd11f57816
Reviewed-on: https://go-review.googlesource.com/16160Reviewed-by: David Crawshaw <crawshaw@golang.org>