RISC-V: Add hooks to use the gdb xml files.

RISC-V: Add hooks to use the gdb xml files.

The gdb CSR xml file has registers in documentation order, not numerical
order, so we need a table to map the register numbers.  This also adds
fairly standard gdb hooks to access xml specified registers.

notice:
    The fpu xml from gdb 8.3 has unused register #, 65 and make first
    csr register # become 69. We register extra register on gdb to correct
    csr offset calculation

Signed-off-by: Jim Wilson <jimw@sifive.com>
Signed-off-by: Chih-Min Chao <chihmin.chao@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>