runtime: factor signal stack code out of sigtrampgo
runtime: factor signal stack code out of sigtrampgo

This reduces the required nosplit stack size, which permits building
on Solaris with -gcflags=all=-N -l.

Fixes #35046

Change-Id: Icb3a421bb791c73e2f670ecfadbe32daea79789f
Reviewed-on: https://go-review.googlesource.com/c/go/+/202446
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

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