Add a mono-hwcap module to utils.

This is an attempt to unify all the ad-hoc CPU feature
detection we do all over the codebase (mostly in Mini
back ends). It will also be used to detect and use SMP
instructions on older ARM devices (we need this info in
utils/, not just in mini/).

Each architecture has a mono-hwcap-$TARGET.h header if
it has specified one. So for example, if you want to
find out if a VFP unit is available on ARM, you would
include mono-hwcap-arm.h and read mono_hwcap_arm_has_vfp.
Remember to guard inclusions of these files with a check
along the lines of:

    #ifdef TARGET_ARM
    #include "mono/utils/mono-hwcap-arm.h"
    /* ... */
    #endif

Architectures that don't specify a file simply get an
empty implementation with no feature variables. This
empty implementation is also used if Mono itself is
compiled as a cross compiler.

Initialization of the mono-hwcap module is done in the
mini_init () function in mini/mini.c, very early in the
startup process. So in general, calling it is not needed.