debugging: modify labs to use directly buildroot host gdb

[Tropicao]

We currently advise trainees to download gdb-multiarch from their distro package manager in order to debug applications during the lab.
We could skip this by using directly host-gdb provided by buildroot

This only needs to

• provide proper instructions to use host-gdb from BR (eg: define CROSS_COMPILE, define GDB=${CROSS_COMPILE}gdb, then use ${GDB} instead of gdb-multiarch
• re-test gdb labs, both the userspace one and the kernel one

[Tropicao]

Previous attempts to do this switch led to gdb being unusable in some environments: for example, using arrows to navigate commands history print some escape sequences in gdb console
Thorough testing will need to be done, especially on advised distributions in training labs (Ubuntu 20.04/22.04/24.04, depending on update order)
Maybe keep gdb-multiarch as an alternative ?

[Tropicao]

See also: bootlin/toolchains-builder#30