Changes include:
1. open compilation option -msan for arm64
2. modify doc to explain -msan is also supported on linux/arm64
3. wrap msan lib API in msan_arm64.s
4. use libc for sigaction syscalls when cgo is enabled
5. use libc for mmap syscalls when cgo is enabled

Change-Id: I26ebe61ff7ce1906125f54a0182a720f9d58ec11
Reviewed-on: https://go-review.googlesource.com/109255
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>