RISC-V: `riscv_hwprobe`-based feature detection on Linux / Android

This commit implements `riscv_hwprobe`-based feature detection as available
on newer versions of the Linux kernel.  It also queries whether the vector
extensions are enabled using `prctl` but this is not supported on QEMU's
userland emulator (as of version 9.2.3) and use the auxiliary vector
as a fallback.

Currently, all extensions discoverable from the Linux kernel version 6.14
and related extension groups (except "Supm", which reports the existence of
`prctl`-based pointer masking control and too OS-dependent) are implemented.

Co-Authored-By: Taiki Endo <te316e89@gmail.com>

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