package/flann: re-enable package on microblaze

package/flann: re-enable package on microblaze

With Microblaze gcc version <= 9.x, the build of flannel fails due to
gcc bug 68485: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68485.

To avoid this, the flann package has a !BR2_microblaze
dependency. However, gcc bug 68485 only triggers when optimization is
enabled, so we can work around the issue by passing -O0, which is what
we do in other Buildroot packages to work around this bug.

So, this commit passes -O0 when BR2_TOOLCHAIN_HAS_GCC_BUG_68485, and
re-enables flann on Microblaze.

Note that the comment was talking about gcc bug 69401, but this gcc
bug is a duplicate of 68485. Since all Buildroot packages now use the
reference to gcc bug 68485 and the option is named
BR2_TOOLCHAIN_HAS_GCC_BUG_68485, we use this naming as well for flann.

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>