curve25519, poly1305: mark constants as RODATA

Fixes tests when using Go tip.  Without this the link steps
fails with errors like:

missing Go type information for global symbol: google3/third_party/golang/go_crypto/curve25519/curve25519.REDMASK51 size 8

LGTM=agl
R=agl
CC=golang-codereviews
https://golang.org/cl/156810043

curve25519/const_amd64.s

@@ -7,14 +7,16 @@
 
 // +build amd64,!gccgo
 
+#include "textflag.h"
+
 DATA ·REDMASK51(SB)/8, $0x0007FFFFFFFFFFFF
-GLOBL ·REDMASK51(SB), $8
+GLOBL ·REDMASK51(SB), RODATA, $8
 
 DATA ·_121666_213(SB)/8, $996687872
-GLOBL ·_121666_213(SB), $8
+GLOBL ·_121666_213(SB), RODATA, $8
 
 DATA ·_2P0(SB)/8, $0xFFFFFFFFFFFDA
-GLOBL ·_2P0(SB), $8
+GLOBL ·_2P0(SB), RODATA, $8
 
 DATA ·_2P1234(SB)/8, $0xFFFFFFFFFFFFE
-GLOBL ·_2P1234(SB), $8
+GLOBL ·_2P1234(SB), RODATA, $8

poly1305/const_amd64.s

@@ -7,39 +7,41 @@
 
 // +build amd64,!gccgo
 
+#include "textflag.h"
+
 DATA ·SCALE(SB)/8, $0x37F4000000000000
-GLOBL ·SCALE(SB), $8
+GLOBL ·SCALE(SB), RODATA, $8
 DATA ·TWO32(SB)/8, $0x41F0000000000000
-GLOBL ·TWO32(SB), $8
+GLOBL ·TWO32(SB), RODATA, $8
 DATA ·TWO64(SB)/8, $0x43F0000000000000
-GLOBL ·TWO64(SB), $8
+GLOBL ·TWO64(SB), RODATA, $8
 DATA ·TWO96(SB)/8, $0x45F0000000000000
-GLOBL ·TWO96(SB), $8
+GLOBL ·TWO96(SB), RODATA, $8
 DATA ·ALPHA32(SB)/8, $0x45E8000000000000
-GLOBL ·ALPHA32(SB), $8
+GLOBL ·ALPHA32(SB), RODATA, $8
 DATA ·ALPHA64(SB)/8, $0x47E8000000000000
-GLOBL ·ALPHA64(SB), $8
+GLOBL ·ALPHA64(SB), RODATA, $8
 DATA ·ALPHA96(SB)/8, $0x49E8000000000000
-GLOBL ·ALPHA96(SB), $8
+GLOBL ·ALPHA96(SB), RODATA, $8
 DATA ·ALPHA130(SB)/8, $0x4C08000000000000
-GLOBL ·ALPHA130(SB), $8
+GLOBL ·ALPHA130(SB), RODATA, $8
 DATA ·DOFFSET0(SB)/8, $0x4330000000000000
-GLOBL ·DOFFSET0(SB), $8
+GLOBL ·DOFFSET0(SB), RODATA, $8
 DATA ·DOFFSET1(SB)/8, $0x4530000000000000
-GLOBL ·DOFFSET1(SB), $8
+GLOBL ·DOFFSET1(SB), RODATA, $8
 DATA ·DOFFSET2(SB)/8, $0x4730000000000000
-GLOBL ·DOFFSET2(SB), $8
+GLOBL ·DOFFSET2(SB), RODATA, $8
 DATA ·DOFFSET3(SB)/8, $0x4930000000000000
-GLOBL ·DOFFSET3(SB), $8
+GLOBL ·DOFFSET3(SB), RODATA, $8
 DATA ·DOFFSET3MINUSTWO128(SB)/8, $0x492FFFFE00000000
-GLOBL ·DOFFSET3MINUSTWO128(SB), $8
+GLOBL ·DOFFSET3MINUSTWO128(SB), RODATA, $8
 DATA ·HOFFSET0(SB)/8, $0x43300001FFFFFFFB
-GLOBL ·HOFFSET0(SB), $8
+GLOBL ·HOFFSET0(SB), RODATA, $8
 DATA ·HOFFSET1(SB)/8, $0x45300001FFFFFFFE
-GLOBL ·HOFFSET1(SB), $8
+GLOBL ·HOFFSET1(SB), RODATA, $8
 DATA ·HOFFSET2(SB)/8, $0x47300001FFFFFFFE
-GLOBL ·HOFFSET2(SB), $8
+GLOBL ·HOFFSET2(SB), RODATA, $8
 DATA ·HOFFSET3(SB)/8, $0x49300003FFFFFFFE
-GLOBL ·HOFFSET3(SB), $8
+GLOBL ·HOFFSET3(SB), RODATA, $8
 DATA ·ROUNDING(SB)/2, $0x137f
-GLOBL ·ROUNDING(SB), $2
+GLOBL ·ROUNDING(SB), RODATA, $2