configure: gate our use of GDB to 8.3.1 or above

configure: gate our use of GDB to 8.3.1 or above

The support of socket based debugging which we need for linux-user
testing is only really stable as of 8.3.1 so lets gate our use of GDB
on having a relatively modern version.

For direct testing you can just point to a locally compiled version of
gdb via configure, e.g.:

  ../../configure --gdb=$HOME/src/binutils-gdb.git/builds/all/install/bin/gdb

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210108224256.2321-4-alex.bennee@linaro.org>