runtime: introduce CPU access functions on windows

This CL introduces new methods for 'context' type, so we can
manipulate its values in an architecture independent way.

Use new methods to replace both 386 and amd64 versions of
dosigprof with single piece of code.

There is more similar code to be converted in the following CLs.

Also remove os_windows_386.go and os_windows_amd64.go. These
contain unused functions.

Change-Id: I28f76aeb97f6e4249843d30d3d0c33fb233d3f7f
Reviewed-on: https://go-review.googlesource.com/2790


Reviewed-by: Minux Ma <minux@golang.org>