target/i386: Add GDB XML description for SSE registers

Add an XML description for SSE registers (XMM+MXCSR) for both X86
and X86-64 architectures in the GDB stub:
- configure: Define gdb_xml_files for the X86 targets (32 and 64bit).
- gdb-xml/i386-32bit-sse.xml & gdb-xml/i386-64bit-sse.xml: The XML files
that contain a description of the XMM + MXCSR registers.
- gdb-xml/i386-32bit.xml & gdb-xml/i386-64bit.xml: wrappers that include
the XML file of the core registers and the other XML file of the SSE registers.
- target/i386/cpu.c: Modify the gdb_core_xml_file to the new XML wrapper,
  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>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>