runtime: cast SIGSTKSZ to uintptr
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 <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

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