runtime: export getm function

runtime: export getm function

    Use go:linkname to export the getm function. This makes it visible to
    runtime/testdata/testprogcgo/dropm_stub.go, which uses it as part of
    the TestEnsureDropM test in runtime/crash_cgo_test.go. That test is
    not run today, but it will be soon.

    Reviewed-on: https://go-review.googlesource.com/46462

From-SVN: r249576