cmd/cgo: make the gccgo init function no_split_stack
cmd/cgo: make the gccgo init function no_split_stack

This works around what appears to be a bug in current clang (2018-11-09).
Details are in the comment in the code.

Change-Id: Ib4783b6c03d531c69ebc4cb0ac023bea5bee7d40
Reviewed-on: https://go-review.googlesource.com/c/148819
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

nawawi/go,CAFxX/go,CAFxX/go,josharian/go,golang/go,golang/go,golang/go,akutz/go,nawawi/go,cloudflare/go,golang/go,josharian/go,golang/go,CAFxX/go,josharian/go,akutz/go,cloudflare/go,nawawi/go,nawawi/go,akutz/go,josharian/go,josharian/go,nawawi/go,akutz/go,CAFxX/go,josharian/go,cloudflare/go,akutz/go,nawawi/go,cloudflare/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,cloudflare/go,golang/go,akutz/go,josharian/go,akutz/go,nawawi/go,CAFxX/go,cloudflare/go,josharian/go,CAFxX/go,nawawi/go,CAFxX/go,akutz/go,golang/go