aarch64: Make cpu-features definitions not Linux-specific

These describe generic AArch64 CPU features, and are not tied to a
kernel-specific way of determining them. We can share them between
the Linux and Hurd AArch64 ports.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-ID: <20240103171502.1358371-13-bugaevc@gmail.com>