runtime, runtime/cgo: use libc for sigaction syscalls when cgo is enabled on loong64

runtime, runtime/cgo: use libc for sigaction syscalls when cgo is enabled on loong64

This ensures that runtime's signal handlers pass through the TSAN and
MSAN libc interceptors and subsequent calls to the intercepted
sigaction function from C will correctly see them.

Change-Id: I243a70d9dcb6d95a65c8494d5f9f9f09a316c693
Reviewed-on: https://go-review.googlesource.com/c/go/+/654995
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>