runtime: cast SIGSTKSZ to uintptr

In newer versions of glibc it is long, which causes a signed
comparison warning.

Fixes https://gcc.gnu.org/PR99458

Change-Id: I9fca4a971455a1361f5435d0ecb80e1a5096486c
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/299849
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>