target/xtensa: use correct number of registers in gdbstub

target/xtensa: use correct number of registers in gdbstub

System emulation should provide access to all registers, userspace
emulation should only provide access to unprivileged registers.
Record register flags from GDB register map definition, calculate both
num_regs and num_core_regs if either is zero. Use num_regs in system
emulation, num_core_regs in userspace emulation gdbstub.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>