tests/tcg: finesse the registers check for "hidden" regs

The reason the ppc64 and s390x test where failing was because gdb
hides them although they are still accessible via regnum. We can
re-arrange the test a little bit and include these two arches in our
test.

We also need to be a bit more careful handling remote-registers as the
format isn't easily parsed with pure white space separation. Once we
fold types like "long long" and "long double" into a single word we
can now assert all registers are either listed or elided.

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>
Cc: Luis Machado <luis.machado@arm.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231121153606.542101-1-alex.bennee@linaro.org>