go/build is changed to list the .m files in a package, and match them for build constraints, adding them to a new field: Package.MFiles.

The go tool is changed to support building .m files and linking in the results during CGO and SWIG builds. This means packages that create a C interface to calls Objective-C code from go are now go-gettable without producing and distributing .syso files. This change is analogous to the one in Go 1.2 made to support C++ built code.

This change doesn't support .mm files (Objective C++).

Also added support for these MFiles to go list's -json mode.

Fixes #6536.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/60590044