runtime: build testprogcgo with -pthread

Building this test with gccgo requires an explicit -pthread option to
be passed to the C compiler, so that it links against -lpthread.
This test is not built today, but it will be soon with a future patch.

Change-Id: I678bad1f486aba206aed44f91e9bf5c9499e2e4c
Reviewed-on: https://go-review.googlesource.com/46451
Reviewed-by: Than McIntosh <thanm@google.com>
1 file changed