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 <thanm@google.com>
1 file changed