Debugger: add AVX support

- Unify storage of "FPU" registers between debugger and signal handler
to use xsave format on both sides
- Handle YMM registers in Debugger (they are the same as XMM, but wider)

Tested:
- The system still boots with and without AVX
- The hello_avx test program can be debugged and the full value of YMM is visible

This changes the API of vregs in signal.h but not the ABI (structure are
declared differently but memory layout is the same). This changes the
API and ABI of arch_debugger.h for x86_64, but I don't think anything
outside Haiku uses it (did we ever have a 64bit compatible gdb?)

Change-Id: If93680ffa0339c19bab517876b4e029f5d66b240
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3038
Reviewed-by: Rene Gollent <rene@gollent.com>

Debugger: Split into core library and application.

- Add subfolder src/kits/debugger which contains the debugger's core
functionality and lower layers. Correspondingly add headers/private/debugger
for shared headers to be used by clients such as the Debugger application
and eventual remote_debug_server. Adjust various files to account for
differences as a result of the split and moves.
- Add libdebugger.so to minimal Jamfile.