arch-riscv: CSR registers support in RISC-V remote GDB.

arch-riscv: CSR registers support in RISC-V remote GDB.

Note:
Some less frequently needed CSR registers (e.g. hpm and pmp registers)
are commented out on purpose. Instructions to add them back are
described in remote_gdb.hh comments. This is to avoid spamming the
remote GDB log when using `info reg all`.

Changes:
1. Added GDB XML files to the ext/ directory (mostly from QEMU)
2. Modified RiscvGdbRegCache
	- struct r: added CSR registers
	- getRegs, setRegs: reading / setting CSR registers
3. Modified RemoteGDB
	- availableFeatures: indicate support for XML registers
	- getXferFeaturesRead: return XML blobs

Change-Id: Ica03b63edb3f0c9b6a7789228b995891dbfb26b2
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38955
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>