arch-riscv: fix GDB register cache

arch-riscv: fix GDB register cache

Fixes the definition of the RISC-V GDB register cache.  The latest
version, of RISC-V gdb, commit c3eb4078520dad8234ffd7fbf893ac0da23ad3c8,
appears to only accept the 32 integer registers + the PC in the 'g'
packet.

This functions with the Linux toolchain (riscv64-unknown-linux-gnu-*),
but works best with the Newlib toolchain (riscv64-unknown-elf-*).

Change-Id: Ie35ea89a45870fb634e6c68236261bde27c86e41
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20028
Reviewed-by: Alec Roelke <alec.roelke@gmail.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Alec Roelke <alec.roelke@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>