gdb: correct version selection on arc/microblaze if host version isn't enabled

Fixes http://autobuild.buildroot.net/results/858/8580840f6e41f198d2cfc0609e14765dbc2e7322/



kconfig uses the first 'default ... if' line that matches, so ensure the
arch specific ones come before the generic 7.5.1 default, otherwise
arc/microblaze ends up with 7.5.1 instead of their custom version.
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>