[lldb] [Process/FreeBSDRemote] Introduce arm64 support

Introduce arm64 support in the FreeBSDRemote plugin.  The code
is roughly based on Linux and reuses the same POSIX RegisterInfos
(but the buffers need to be a few bytes larger due to stricter struct
member alignment in FreeBSD structures -- luckily, they do not affect
the actual member offsets).  It supports reading and writing
general-purpose and FPU registers.  SVE and hardware watchpoint support
is missing due to the limitations of FreeBSD ptrace(2) API.

Differential Revision: https://reviews.llvm.org/D95297