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

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-27204e41643f4549c5ed97d48e17912f64c0aa71
+55b2b2dee55bd495a770347daaf858634057ecb0
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/proc.go

@@ -37,6 +37,9 @@ import (
 //go:linkname globrunqput runtime.globrunqput
 //go:linkname pidleget runtime.pidleget
 
+// Exported for test (see runtime/testdata/testprogcgo/dropm_stub.go).
+//go:linkname getm runtime.getm
+
 // Function called by misc/cgo/test.
 //go:linkname lockedOSThread runtime.lockedOSThread