CL 198544 broke the linux/arm64 build because it declares emptyfunc for
GOARCH=arm64, but only freebsd/arm64 defines it. Make it a static
assembly function specific for freebsd/arm64 and remove the stub.

Fixes #35160

Change-Id: I5fd94249b60c6fd259c251407b6eccc8fa512934
Reviewed-on: https://go-review.googlesource.com/c/go/+/203418Reviewed-by: Bryan C. Mills <bcmills@google.com>