cpuinfo: Check for extensions using riscv_hwprobe

[OFFTKP]

Update Cpuinfo to use the linux syscall riscv_hwprobe to check for extensions instead of the auxiliary vector value.

Each case is written out explicitly because in the future more extensions might be added and the syscall may include more bitfields when the current one (RISCV_HWPROBE_KEY_IMA_EXT_0) runs out of bits.

https://github.com/torvalds/linux/blob/master/arch/riscv/include/uapi/asm/hwprobe.h

[lioncash]

Mostly minor stylistic comments, but nice changes overall. I really like getting nix specific headers out of the cpuinfo header, so projects don't implicitly have those dumped in for feature testing

[OFFTKP]

All done!

[OFFTKP]

I should mention that this syscall exists from Linux 6.4 onwards, maybe it should fallback to the old implementation if the syscall fails

Edit: Made it fallback to the old implementation if the syscall returns error due to not existing