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

[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.