test: fix nosplit test for noopt build

Noopt builds get a larger stack guard.  This test must take that into account.

Change-Id: I1b5cbafdbbfee8c369ae1bebd0b900524ebf0d7d
Reviewed-on: https://go-review.googlesource.com/9610Reviewed-by: Russ Cox <rsc@golang.org>

test/nosplit.go

@@ -292,9 +292,16 @@ TestCases:
 
 				// The limit was originally 128 but is now 512.
 				// Instead of rewriting the test cases above, adjust
-				// the first stack frame to use up the extra 32 bytes.
+				// the first stack frame to use up the extra bytes.
 				if i == 0 {
 					size += 512 - 128
+					// Noopt builds have a larger stackguard.
+					// See ../cmd/dist/buildruntime.go:stackGuardMultiplier
+					for _, s := range strings.Split(os.Getenv("GO_GCFLAGS"), " ") {
+						if s == "-N" {
+							size += 640
+						}
+					}
 				}
 
 				if size%ptrSize == 4 {