fix cloudflare problem in go 1.11

Author: CoderZhi

crypto/bn256/cloudflare/gfp_amd64.s

@@ -110,7 +110,7 @@ TEXT ·gfpMul(SB),0,$160-24
 	MOVQ b+16(FP), SI
 
 	// Jump to a slightly different implementation if MULX isn't supported.
-	CMPB runtime·support_bmi2(SB), $0
+	CMPB ·hasBMI2(SB), $0
 	JE   nobmi2Mul
 
 	mulBMI2(0(DI),8(DI),16(DI),24(DI), 0(SI))

crypto/bn256/cloudflare/gfp_decl.go

@@ -5,6 +5,12 @@ package bn256
 // This file contains forward declarations for the architecture-specific
 // assembly implementations of these functions, provided that they exist.
 
+import (
+    "golang.org/x/sys/cpu"
+)
+
+var hasBMI2 = cpu.X86.HasBMI2
+
 // go:noescape
 func gfpNeg(c, a *gfP)