Add an implementation of .mcount on arm64

To support cc -pg on arm64 we need to implement .mcount. As clang and
gcc think it is function like it just needs to load the arguments
to _mcount and call it.

On gcc the first argument is passed in x0, however this is missing on
clang so we need to load it from the stack. As it's the caller return
address this will be at a known location.

PR: 262709
Reviewed by: emaste (earlier version)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34634