package/ffmpeg: re-enable package if gcc bug 85180 is present

With Microblaze Gcc version < 8.x the build hangs due to gcc bug
85180: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85180

. To avoid
this problem, until now, ffmpeg could not be selected on
Microblaze. However, this problem only happens due to optimization,
and can worked around by forcing -O0.

So, when BR2_TOOLCHAIN_HAS_GCC_BUG_85180=y, this commit:
- adds --disable-optimizations to FFMPEG_CONF_OPTS
- passes -O0 to CFLAGS in FFMPEG_CONF_ENV

Then, we remove 'depends on !BR2_microblaze' from Config.in to
re-enable the package.

Note that the comment was talking about gcc bug 71124, but this gcc
bug is a duplicate of 85180. Since all Buildroot packages now use the
reference to gcc bug 85180 and the option is named
BR2_TOOLCHAIN_HAS_GCC_BUG_85180, we use this naming as well for
ffmpeg.

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
[Thomas: tweak CFLAGS logic, improve commit log]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>