overlays: xtensa_espressif_esp32: gdb: Relocate gdbserver files

This commit relocates the gdbserver `xtensa-xtregs.c`, which was
previously located under `gdb/gdbserver` to `gdbserver` as required by
the GDB 12.1.

Also note that the `xtensa-xtregs.c` file has been renamed to
`xtensa-xtregs.cc`.

The `xtensa-regmap.c` file is removed because it is not used anywhere.

Signed-off-by: Stephanos Ioannidis <[email protected]>

Author: stephanosio