runtime: clean up now unused pushCallSupported

All platforms now support pushCall, hence remove the now unnecessary
pushCallSupported flag/guard.

Change-Id: I99e4be73839da68a742f3c239bae9ce2f8764624
Reviewed-on: https://go-review.googlesource.com/c/go/+/228497
Run-TryBot: Joel Sing <joel@sing.id.au>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/runtime/signal_unix.go

@@ -336,7 +336,7 @@ func doSigPreempt(gp *g, ctxt *sigctxt) {
 	atomic.Store(&gp.m.signalPending, 0)
 }
 
-const preemptMSupported = pushCallSupported
+const preemptMSupported = true
 
 // preemptM sends a preemption request to mp. This request may be
 // handled asynchronously and may be coalesced with other requests to
@@ -345,11 +345,6 @@ const preemptMSupported = pushCallSupported
 // safe-point, it will preempt the goroutine. It always atomically
 // increments mp.preemptGen after handling a preemption request.
 func preemptM(mp *m) {
-	if !pushCallSupported {
-		// This architecture doesn't support ctxt.pushCall
-		// yet, so doSigPreempt won't work.
-		return
-	}
 	if GOOS == "darwin" && GOARCH == "arm64" && !iscgo {
 		// On darwin, we use libc calls, and cgo is required on ARM64
 		// so we have TLS set up to save/restore G during C calls. If cgo is