runtime: fix nacl amd64p32 flakiness

newproc takes two extra pointers, not two extra registers.
On amd64p32 (nacl) they are different.

We diagnosed this before the 1.3 cut but the tree was frozen.
I believe this is causing the random problems on the builder.

Fixes #8199.

TBR=r
CC=golang-codereviews
https://golang.org/cl/102710043

src/pkg/runtime/traceback_x86.c

@@ -214,7 +214,7 @@ runtime·gentraceback(uintptr pc0, uintptr sp0, uintptr lr0, G *gp, int32 skip,
 		// the SP is two words lower than normal.
 		sparg = frame.sp;
 		if(wasnewproc)
-			sparg += 2*sizeof(uintreg);
+			sparg += 2*sizeof(uintptr);
 
 		// Determine frame's 'continuation PC', where it can continue.
 		// Normally this is the return address on the stack, but if sigpanic