runtime: increase nosplit area to 192

In CL 131450043, which raised it to 160,
I'd raise it to 192 if necessary.
Apparently it is necessary on windows/amd64.

One note for those concerned about the growth:
in the old segmented stack world, we wasted this much
space at the bottom of every stack segment.
In the new contiguous stack world, each goroutine has
only one stack segment, so we only waste this much space
once per goroutine. So even raising the limit further might
still be a net savings.

Fixes windows/amd64 build.

TBR=r
CC=golang-codereviews
https://golang.org/cl/132480043

src/pkg/runtime/stack.go

@@ -86,7 +86,7 @@ const (
 	// 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 = 96
+	stackSmall = 64
 
 	// The maximum number of bytes that a chain of NOSPLIT
 	// functions can use.