Make all Mini back ends use mono-hwcap for CPU feature detection.

Also some related cleanup as a result of this.
Check for sys/auxv.h and only use it if available.
Remove the JIT-ed CPUID hack from mono-hwcap-x86.

A proper workaround for the 32-bit PIC issue exists: Using xchg to
unclobber EBX which is used by the compiler. This means we can just
use inline assembly to issue the instruction, eliminating the hack
where we JIT-compiled a function to do the CPUID.

This patch also makes it so that we use MSVC's __cpuid intrinsic
on both 32-bit and 64-bit x86 since it's available on both.
Remove now-unneeded mono_aot_only guard around mono_hwcap_init ().

We no longer use the code manager in any of the hwcap implementations,
so we don't need to disable the hwcap infrastructure in full-aot mode.
Add a MONO_VERBOSE_HWCAP environment variable.

This causes detected hardware features to be printed to stdout.