target/i386: Add GDB XML register description support

This patch implements XML target description support for X86 and X86-64
architectures in the GDB stub, as the way with ARM and PowerPC:
- gdb-xml/32bit-core.xml & gdb-xml/64bit-core.xml: Adding the XML target
  description files, these files are picked from GDB source code.
- configure: Define gdb_xml_files for X86 targets.
- target/i386/cpu.c: Define gdb_core_xml_file and gdb_arch_name to add
  XML awareness for this architecture, modify the gdb_num_core_regs to
  fit the registers number defined in each XML file.

Signed-off-by: Abdallah Bouassida <abdallah.bouassida@lauterbach.com>
Message-Id: <2b3c8119-1602-28c7-eab4-296593877103@lauterbach.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>