runtime: subtract one from ip when determining abort

On windows/arm, the abort is given from one byte off of the function
address, perhaps because Windows wants to simulate x86/amd64 modes, or
because it's jumping from thumb mode. This is not the case with
windows/arm64, though.

This prevents a failure in the builders with the TestAbort test:

    crash_test.go:727: output contains BAD:
        panic: runtime error: invalid memory address or nil pointer dereference [recovered]
                panic: BAD: recovered from abort
        [signal 0xc0000005 code=0x0 addr=0x0 pc=0x6a5721]

Change-Id: I8939c60611863cc0c325e179a772601acea9fd4a
Reviewed-on: https://go-review.googlesource.com/c/go/+/296153
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/runtime/signal_windows.go

@@ -45,9 +45,10 @@ func initExceptionHandler() {
 //go:nosplit
 func isAbort(r *context) bool {
 	pc := r.ip()
-	if GOARCH == "386" || GOARCH == "amd64" {
+	if GOARCH == "386" || GOARCH == "amd64" || GOARCH == "arm" {
 		// In the case of an abort, the exception IP is one byte after
-		// the INT3 (this differs from UNIX OSes).
+		// the INT3 (this differs from UNIX OSes). Note that on ARM,
+		// this means that the exception IP is no longer aligned.
 		pc--
 	}
 	return isAbortPC(pc)