gdbserver: fix gdbserver builds after expedite_regs changes

After this commit:

  commit 6a65998a
  Date:   Mon Sep 11 12:42:00 2023 +0100

      Convert tdesc's expedite_regs to a string vector

The risc-v, loongarch, and csky gdbserver builds were broken.  A use
of target_desc::expedite_regs (for each architecture)  was not updated
to take account of the type change.

I've tested that this fixes the risc-v build.  I haven't tested the
other architectures, but they should be fine.