tests/tcg: add an explicit gdbstub register tester

tests/tcg: add an explicit gdbstub register tester

We already do a couple of "info registers" for specific tests but this
is a more comprehensive multiarch test. It also has some output
helpful for debugging the gdbstub by showing which XML features are
advertised and what the underlying register numbers are.

My initial motivation was to see if there are any duplicate register
names exposed via the gdbstub while I was reviewing the proposed
register interface for TCG plugins.

Mismatches between the xml and remote-desc are reported for debugging
but do not fail the test.

We also skip the tests for the following arches for now until we can
investigate and fix any issues:

  - s390x (fails to read v0l->v15l, not seen in remote-registers)
  - ppc64 (fails to read vs0h->vs31h, not seen in remote-registers)

Cc: Akihiko Odaki <akihiko.odaki@daynix.com>
Cc: Luis Machado <luis.machado@linaro.org>
Cc: Ilya Leoshkevich <iii@linux.ibm.com>
Cc: qemu-s390x@nongnu.org
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Daniel Henrique Barboza <danielhb413@gmail.com>
Cc: qemu-ppc@nongnu.org
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231106185112.2755262-7-alex.bennee@linaro.org>