package/gstreamer/gst-ffmpeg: re-enable package when gcc bug 85180 is present

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

. This package has
been excluded from building if the toolchain is affected by gcc bug
85180. To be consistent with how we deal with this issue in other
packages, we re-enable the package and instead work around the issue
by building with -O0, since gcc bug 85180 manifests itself only when
optimization is enabled.

To achieve this:

- add --disable-optimizations to EXTRA_OPTS, which gets passed down to
  the built-in libav library
- pass -O0 to CFLAGS in CONF_ENV

Then we remove 'depends on !BR2_TOOLCHAIN_HAS_GCC_BUG_85180' and its
comment if not available from Config.in

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>