[Linux] Minimally support probing CPUs with non-Loongson PRID

[xen0n]

https://github.com/torvalds/linux/blob/5bd7ef53ffe5ca580e93e74eb8c81ed191ddc4bd/arch/loongarch/kernel/cpu-probe.c#L312-L316

This means any non-Loongson LoongArch design will not correctly probe, because they don't get a PRID with a company ID of Loongson.

According to the LoongArch spec, at least the CPUCFG data is standardized across all compliant LoongArch implementations, so we should be able to derive essential feature bits (such as the ELF platform -- bitness info) without first checking PRID.

[xen0n]

cc @chenhuacai -- I might do this early next year, targetting v6.8 or v6.9 depending on my free time.