With the new buildmodes c-archive and c-shared, it is possible for a
cgo call to come in early in the lifecycle of a Go program. Calls
before the runtime has been initialized are caught by
_cgo_wait_runtime_init_done. However a call can come in after the
runtime has initialized, but before the program's package init
functions have finished running.

To avoid this cgocallback checks m.ncgo to see if we are on a thread
running Go. If not, we may be a foreign thread and it blocks until
main_init is complete.

Change-Id: I7a9f137fa2a40c322a0b93764261f9aa17fcf5b8
Reviewed-on: https://go-review.googlesource.com/8897Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>