RISC-V: Linux 6.15 riscv_hwprobe support

[a4lg]

This commit adds support for riscv_hwprobe on the Linux kernel 6.15. It adds feature detection of 8 extensions (4 of them are new in this).

Existing RISC-V Extensions:

1. Zicntr
2. Zihpm
3. Zalrsc
4. Zaamo

New RISC-V Extensions:

5. Zicbom
6. Zfbfmin
7. Zvfbfmin
8. Zvfbfwma

Note: four new extensions are already added to beta (cf. rust-lang/rust#140507) and checking cfg should be okay.

[rustbot]

r? @Amanieu

rustbot has assigned @Amanieu.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer