sysutils/cpuid2cpuflags: make the port build and work on ARM

- Replace getauxval(3), a non-standard glibc extension, with
  our native elf_aux_info(3) call
- Return features supported by the specific machine processor
  architecture (uname -p) rather than the hardware platform
  (uname -m), because the latter is too vague on FreeBSD/arm

PR:		258445
Reported by:	pkg-fallout