MacOS arm64 support

[mburbidg]

I'm trying to use memberlist on a Mac M1 and I'm getting the following errors when building.

golang.org/x/sys/unix

../go/pkg/mod/golang.org/x/sys@v0.0.0-20190924154521-2837fb4f24fe/unix/zsyscall_darwin_arm64.go:28:3: //go:linkname must refer to declared function or variable
../go/pkg/mod/golang.org/x/sys@v0.0.0-20190924154521-2837fb4f24fe/unix/zsyscall_darwin_arm64.go:43:3: //go:linkname must refer to declared function or variable
../go/pkg/mod/golang.org/x/sys@v0.0.0-20190924154521-2837fb4f24fe/unix/zsyscall_darwin_arm64.go:59:3: //go:linkname must refer to declared function or variable
../go/pkg/mod/golang.org/x/sys@v0.0.0-20190924154521-2837fb4f24fe/unix/zsyscall_darwin_arm64.go:75:3: //go:linkname must refer to declared function or variable
../go/pkg/mod/golang.org/x/sys@v0.0.0-20190924154521-2837fb4f24fe/unix/zsyscall_darwin_arm64.go:90:3: //go:linkname must refer to declared function or variable
../go/pkg/mod/golang.org/x/sys@v0.0.0-20190924154521-2837fb4f24fe/unix/zsyscall_darwin_arm64.go:105:3: //go:linkname must refer to declared function or variable
../go/pkg/mod/golang.org/x/sys@v0.0.0-20190924154521-2837fb4f24fe/unix/zsyscall_darwin_arm64.go:121:3: //go:linkname must refer to declared function or variable
../go/pkg/mod/golang.org/x/sys@v0.0.0-20190924154521-2837fb4f24fe/unix/zsyscall_darwin_arm64.go:136:3: //go:linkname must refer to declared function or variable
../go/pkg/mod/golang.org/x/sys@v0.0.0-20190924154521-2837fb4f24fe/unix/zsyscall_darwin_arm64.go:151:3: //go:linkname must refer to declared function or variable
../go/pkg/mod/golang.org/x/sys@v0.0.0-20190924154521-2837fb4f24fe/unix/zsyscall_darwin_arm64.go:166:3: //go:linkname must refer to declared function or variable
../go/pkg/mod/golang.org/x/sys@v0.0.0-20190924154521-2837fb4f24fe/unix/zsyscall_darwin_arm64.go:166:3: too many errors