runtime: give nosplit functions 32 more bytes of headroom

The Go calling convention uses more stack space than C.
On 64-bit systems we've been right up against the limit
(128 bytes, so only 16 words) and doing awful things to
our source code to work around it. Instead of continuing
to do awful things, raise the limit to 160 bytes.
I am prepared to raise the limit to 192 bytes if necessary,
but I think this will be enough.

Should fix current link-time stack overflow errors on
        - nacl/arm
        - netbsd/amd64
        - openbsd/amd64
        - solaris/amd64
        - windows/amd64

TBR=r
CC=golang-codereviews, iant
https://golang.org/cl/131450043

src/pkg/runtime/stack.h

@@ -94,7 +94,7 @@ enum {
 	// After a stack split check the SP is allowed to be this
 	// many bytes below the stack guard.  This saves an instruction
 	// in the checking sequence for tiny frames.
-	StackSmall = 128,
+	StackSmall = 96,
 
 	// The maximum number of bytes that a chain of NOSPLIT
 	// functions can use.

src/pkg/runtime/stack_test.go

@@ -15,7 +15,8 @@ import (
 // See stack.h.
 const (
 	StackGuard = 256
-	StackLimit = 128
+	StackSmall = 96
+	StackLimit = StackGuard - StackSmall
 )
 
 // Test stack split logic by calling functions of every frame size

test/nosplit.go

@@ -242,7 +242,7 @@ TestCases:
 			if line == "" {
 				continue
 			}
-			for _, subline := range strings.Split(line, ";") {
+			for i, subline := range strings.Split(line, ";") {
 				subline = strings.TrimSpace(subline)
 				if subline == "" {
 					continue
@@ -255,6 +255,14 @@ TestCases:
 				}
 				name := m[1]
 				size, _ := strconv.Atoi(m[2])
+
+				// CL 131450043 raised the limit from 128 to 160.
+				// Instead of rewriting the test cases above, adjust
+				// the first stack frame to use up the extra 32 bytes.
+				if i == 0 {
+					size += 32
+				}
+
 				if goarch == "amd64" && size%8 == 4 {
 					continue TestCases
 				}