[lldb][FreeBSD] Add FreeBSD specific AT_HWCAP value (#84147)

While adding register fields I realised that the AUXV values for Linux
and FreeBSD disagree here.

So I've added a FreeBSD specific HWCAP value that I can use from FreeBSD
specific code.

The alternative is translating GetAuxValue calls depending on platform,
which requires that we know what we are at all times.

Another way would be to convert the entries' values when we construct
the AuxVector but the platform specific call that reads the data just
returns a raw array. So adding another layer here is more disruption.