sync/atomic: fix mipsx frame sizes

Found by vet.

Change-Id: Ied3089a2cc8757ae5377fb5fa05bbb385d26ad9c
Reviewed-on: https://go-review.googlesource.com/37307
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/sync/atomic/asm_mipsx.s

@@ -12,13 +12,13 @@ TEXT ·SwapInt32(SB),NOSPLIT,$0-12
 TEXT ·SwapUint32(SB),NOSPLIT,$0-12
 	JMP	runtime∕internal∕atomic·Xchg(SB)
 
-TEXT ·SwapInt64(SB),NOSPLIT,$0-24
+TEXT ·SwapInt64(SB),NOSPLIT,$0-20
 	JMP	runtime∕internal∕atomic·Xchg64(SB)
 
-TEXT ·SwapUint64(SB),NOSPLIT,$0-24
+TEXT ·SwapUint64(SB),NOSPLIT,$0-20
 	JMP	runtime∕internal∕atomic·Xchg64(SB)
 
-TEXT ·SwapUintptr(SB),NOSPLIT,$0-20
+TEXT ·SwapUintptr(SB),NOSPLIT,$0-12
 	JMP	runtime∕internal∕atomic·Xchg(SB)
 
 TEXT ·CompareAndSwapInt32(SB),NOSPLIT,$0-13