configure: move detected gdb to TCG's config-host.mak

When tests/tcg gained it's own config-host.mak we forgot to move the
GDB detection.

Fixes: 544f4a25 (tests/tcg: isolate from QEMU's config-host.mak)
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220929114231.583801-6-alex.bennee@linaro.org>