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>

package/ffmpeg/Config.in

@@ -7,8 +7,6 @@ config BR2_PACKAGE_FFMPEG_ARCH_SUPPORTS
 	depends on !BR2_nios2
 	# No support for ARMv7-M in the ARM assembly logic
 	depends on !BR2_ARM_CPU_ARMV7M
-	# Microblaze build affected by gcc PR71124 (infinite loop)
-	depends on !BR2_microblaze
 	# m68k coldfire causes a build failure, because the check for
 	# atomics (atomic_store) succeeds, which causes ffmpeg to
 	# think atomic intrinsics are available, while they are

package/ffmpeg/ffmpeg.mk

@@ -523,6 +523,14 @@ else ifneq ($(GCC_TARGET_ARCH),)
 FFMPEG_CONF_OPTS += --cpu="$(GCC_TARGET_ARCH)"
 endif
 
+FFMPEG_CFLAGS = $(TARGET_CFLAGS)
+
+ifeq ($(BR2_TOOLCHAIN_HAS_GCC_BUG_85180),y)
+FFMPEG_CONF_OPTS += --disable-optimizations
+FFMPEG_CFLAGS += -O0
+endif
+
+FFMPEG_CONF_ENV += CFLAGS="$(FFMPEG_CFLAGS)"
 FFMPEG_CONF_OPTS += $(call qstrip,$(BR2_PACKAGE_FFMPEG_EXTRACONF))
 
 # Override FFMPEG_CONFIGURE_CMDS: FFmpeg does not support --target and others