runtime: make gsignal stack at least SIGSTKSZ bytes
runtime: make gsignal stack at least SIGSTKSZ bytes

The default stack size for the gsignal goroutine, 32K, is not enough on
ia64.  Make sure that the stack size is at least SIGSTKSZ.

Change-Id: I38ca8ba10bd4df096d7a0fe3c1b6ae6f45381c76
Reviewed-on: https://go-review.googlesource.com/28224
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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