MacOS arm64 support

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

Start by reproducing the build on a Mac M1 and inspect the errors from golang.org/x/sys/unix/zsyscall_darwin_arm64.go. Then check the project's Go dependency and toolchain configuration for the source of the arm64 incompatibility. Done means memberlist builds successfully on macOS arm64 without these linkname errors.

Written by the indexing model from the issue text.