package/qt5/qt5base: work around gcc bug 90620

package/qt5/qt5base: work around gcc bug 90620

On Microblaze, with gcc versions <= 9.x and optimization enabled, the
build fails with:

  internal compiler error: in do_output_reload, at reload1.c:7978

This is due to gcc bug 90620
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90620). The bug shows up
when building qt5base (or other qt5 modules) with optimization but not
when building with -O0. To work around this bug, if
BR2_TOOLCHAIN_HAS_GCC_BUG_90620=y we force using -O0.

Fixes:
http://autobuild.buildroot.net/results/4e5/4e563f5fc38ec681a730a576c5f056bc092db74e/
http://autobuild.buildroot.net/results/348/3482bd950c7ab5608666807a7ab7ea857c767333/
http://autobuild.buildroot.net/results/835/8351e3fbd6a39d05da6a6e7a978329d718dfc109/

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