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.

Change-Id: Ib2eb427bf490f56ceee3eec3a91e5af569ec09cf
Reviewed-on: https://go-review.googlesource.com/46462
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend