Stop defaulting to DWRAF2 in kernel compilation

After the removal of obsolete GDB 6.1.1 from the base system in
1c0ea326aa6d we no longer need to downgrade to DWARF2 debug info.

We will need to ensure that our tools (e.g. ctfconvert) handle DWARF5
prior to it becoming the default in the Clang and GCC versions we use.

Reported by: jhb
Relnotes: Yes
Sponsored by: The FreeBSD Foundation