boot/afboot-stm32: fix build with gcc 10

boot/afboot-stm32: fix build with gcc 10

This commit adds a patch that passes -fno-builtin to fix the build
with gcc 10. The patch has been submitted upstream.

There are no autobuilder failures, since it's a bootloader package,
and therefore not tested by the autobuilders. However, a build failure
was detected when building toolchains and a minimal ARMv7-M system at
https://gitlab.com/bootlin/toolchains-builder/-/jobs/729359529.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>