target/riscv: Validate misa_mxl_max only once

target/riscv: Validate misa_mxl_max only once

misa_mxl_max is now a class member and initialized only once for each
class. This also moves the initialization of gdb_core_xml_file which
will be referenced before realization in the future.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240203-riscv-v11-3-a23f4848a628@daynix.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>