CPU detection does not match kernel support

The current code for detecting CPU features on Linux relies on parsing /proc/cpuinfo.

This poses a number of problems:

• It fails when running on user-mode emulator (such as QEMU or Spike), as we get the CPU infos for the host architecture instead of the emulated ISA.
• On RISC-V, the ISA field comes straight from the Device Tree, and may not reflect actual kernel support.
  • It is also very cumbersome due to the idiosyncratic syntax of RISC-V extensions.
• The current code fails to handle setups with asymmetric CPU capabilities.

Not sure about x86, but on all other platforms with run-time CPU detection (ARM, PPC, RISC-V), the support is supposed to be checked from auxiliary vector auxv. libc provides a convenience wrapper getauxval().