arch-riscv,misc: Fix clang missing override errors

arch-riscv,misc: Fix clang missing override errors

Clang 9 failed to compile RISC-V due to missing overrides in
`src/arch/riscv/remote_gdb.hh`. This commit adds these missing
overrides.

Change-Id: Id0bfc371ca3e3e1b91e9112a837e1862072bf9d2
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40395
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>