cpu: Introduce CPUState::gdb_num_regs and CPUClass::gdb_num_core_regs

CPUState::gdb_num_regs replaces num_g_regs.
CPUClass::gdb_num_core_regs replaces NUM_CORE_REGS.

Allows building gdb_register_coprocessor() for xtensa, too.

As a side effect this should fix coprocessor register numbering for SMP.

Acked-by: Michael Walle <michael@walle.cc> (for lm32)
Acked-by: Max Filippov <jcmvbkbc@gmail.com> (for xtensa)
Signed-off-by: Andreas Färber <afaerber@suse.de>