gcc version used for kernel configs doesn't match the one used for builds

[Kleber Sacilotto de Souza]

[Impact]
linux-riscv-5.8 needs to be built with gcc-10 in Focal otherwise it won't boot. This was fixed by commit "UBUNTU: [Packaging] Use gcc-10 for riscv 5.8 kernel build", which adds gcc-10 as "Build-Depends-Arch" and sets a variable "gcc" in 'debian/rules', which is passed down to make by 'debian/rules.d/2-binary-arch.mk'.

The other gcc version dependency is when we generate locally the kernel configs by running 'make syncconfig', which is called by 'debian/scripts/misc/kernelconfig'. Currently, the syncconfig is called using the chroot default compiler, which in Focal is gcc-9. So far there hasn't been any major differences, however with the update of gcc-10 in Groovy and Focal from 10.2.0 to 10.3.0 some KASAN related config options are not available anymore, causing some changes on the configs generated. The problem with non-matching gcc versions being used, is that there will be a mismatch between the configs being generated during the preparation of the kernel and the ones during the build.

[Fix]
The proposed fix is to use the same variable defined for the build, passing it down to 'debian/scripts/misc/kernelconfig' and using it to define the correct cross-compiler to be used.

[Where problems could occur]
If the environment variables are not set correctly the build can break in some environment which was not tested.

[Kleber Sacilotto de Souza (kleber-souza)]

For the record, this is what's causing the config differences between the gcc versions:

With gcc 10.3.0, CC_HAS_KASAN_GENERIC is not available as it depends on the following compiler flag:

config CC_HAS_KASAN_GENERIC
         def_bool $(cc-option, -fsanitize=kernel-address)

'cc-option' will do something similar to the following, which with gcc 10.2.0 riscv64 cross compiler would return success:

$ /usr/bin/riscv64-linux-gnu-gcc-10 -Werror -fsanitize=kernel-address -c -x c /dev/null
$ echo $?
0

But with 10.3.0 this doesn't seem to be supported anymore as a standalone flag, depending now on '-fasan-shadow-offset=':

$ /usr/bin/riscv64-linux-gnu-gcc-10 -Werror -fsanitize=kernel-address -c -x c /dev/null
cc1: error: ‘-fsanitize=kernel-address’ with stack protection is not supported without ‘-fasan-shadow-offset=’ for this target [-Werror]

And all KASAN options get removed altogether.

This doesn't cause any real config change, as we disable CONFIG_KASAN anyway. But for our annotations '-' != 'n', failing the config checks.