toolchain: improve conditions for gcc bug 99140

Gcc bug 99140 has been fixed on gcc 8.x but reappeared on gcc 9.x while
it's been fixed on gcc 10.x+. So let's update
BR2_TOOLCHAIN_HAS_GCC_BUG_99140 accordingly.

Fixes:
http://autobuild.buildroot.net/results/c55/c55f50a8d657695f0d5492c32efa666254cd7f99/



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