configure: bump the minimum gdb version for check-tcg to 9.1

configure: bump the minimum gdb version for check-tcg to 9.1

For SVE, currently the bulk of the GDB TCG tests, we need at least GDB
9.1 to support the "ieee_half" data type we report. This only affects
when GDB tests are run; users can still use lower versions of gdb as
long as they aren't talking to an SVE enabled model. The work around
is to either get a newer gdb or disable SVE for their CPU model.

Reported-by: Claudio Fontana <cfontana@suse.de>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Luis Machado <luis.machado@linaro.org>
Message-Id: <20210202134001.25738-9-alex.bennee@linaro.org>