crypto/bn256: fix issues caused by Go 1.11

crypto/bn256/cloudflare/gfp_amd64.s

crypto/bn256/cloudflare/gfp_decl.go

crypto/bn256/google/bn256.go

crypto/bn256/google/curve.go

crypto/bn256/google/twist.go

vendor/golang.org/x/sys/cpu/cpu.go

vendor/golang.org/x/sys/cpu/cpu_arm.go

vendor/golang.org/x/sys/cpu/cpu_arm64.go

vendor/golang.org/x/sys/cpu/cpu_gc_x86.go

vendor/golang.org/x/sys/cpu/cpu_gccgo.c

vendor/golang.org/x/sys/cpu/cpu_gccgo.go

vendor/golang.org/x/sys/cpu/cpu_mips64x.go

vendor/golang.org/x/sys/cpu/cpu_mipsx.go

vendor/golang.org/x/sys/cpu/cpu_ppc64x.go

vendor/golang.org/x/sys/cpu/cpu_s390x.go

vendor/golang.org/x/sys/cpu/cpu_x86.go

vendor/golang.org/x/sys/cpu/cpu_x86.s

vendor/vendor.json