configure: Avoid use of param. expansion when using gdb_version

configure: Avoid use of param. expansion when using gdb_version

$gdb_version is now used in more than one conditional case and its usage
in such cases may increase in the future. Therefore, avoid using shell
parameter expansion when using it by setting gdb_version to its final
form.

Signed-off-by: Gustavo Romero <gustavo.romero@linaro.org>
Message-Id: <20240804161850.2646299-3-gustavo.romero@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240813202329.1237572-5-alex.bennee@linaro.org>