Change all references to "__INTEL__" to "__i386__".

They are functionally identical, but the former is a BeOS/Haiku-specfic
macro that we include in the compiler specs, and the latter is defined
by GCC.

Fix catching polymorphic type by value

Replace catching polymorphic type std::bad_alloc 'by value'
with 'by reference'.
Pointed by gcc8

libdebugger: Add accessor for cpu state size.

Architecture:
- Store and provide accessor for the size in bytes of the low-level
debug_cpu_state size of the respective target CPU. Adjust subclasses
to pass in the appropriate size information.

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.