Add an implementation of .mcount on arm64.

[andrew]

Summary

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

[andrew]

Add a comment on where frompc comes from

[emaste]

lib/libc/aarch64/Symbol.map

14

I guess adding this to FBSD_1.0 (rather than FBSD_1.7) is appropriate given that this is existing functionality that was just missing from arm64's entry.

[emaste]

Looks OK to me. I think it's worth a comment or commit message note with what we discussed on IRC with respect to caller-saved registers.