poly1305: fix compilation on arm with go tip

Fix compilation of poly1305 using go tip - it currently fails with:

./poly1305_arm.s:124: cannot reference SP without a symbol
./poly1305_arm.s:161: cannot reference SP without a symbol
./poly1305_arm.s:162: cannot reference SP without a symbol
asm: asm: assembly of ./poly1305_arm.s failed

Change-Id: I797dcf3641cc881b6cc192034b693ccf58317987
Reviewed-on: https://go-review.googlesource.com/10307
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>

poly1305/poly1305_arm.s

@@ -121,7 +121,7 @@ poly1305_blocks_armv6_mainloop:
   MOVW g, 8(R13)
   MOVW R11, 12(R13)
   MOVW R12, 0(R13)
-  MOVW R14, 4(SP)
+  MOVW R14, w+4(SP)
   MULLU R0, R5, (R11, g)
   MULALU R4, R6, (R11, g)
   MULALU R3, R7, (R11, g)
@@ -158,8 +158,8 @@ poly1305_blocks_armv6_mainloop:
   MOVW R4>>26, R12
   BIC $0xfc000000, R4, R8
   ADD R12, R6, R9
-  MOVW 44(SP), R12
-  MOVW 40(SP), R14
+  MOVW w+44(SP), R12
+  MOVW w+40(SP), R14
   MOVW R0, R6
   CMP $32, R12
   SUB $16, R12, R12