toolchain: introduce BR2_TOOLCHAIN_HAS_GCC_BUG_68485

GCC hangs while building brotli for the Microblaze Arch:
http://autobuild.buildroot.net/results/d86/d86251974a0a348a64d9a1d1fd7d02dd4aff0792/

Originally reported for gpsd:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68485



Still not fixed. Every Microblaze Gcc version up to and including 9.1
is affected.

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>