MINOR: compiler: move CPU capabilities definition from config.h and complete them

MINOR: compiler: move CPU capabilities definition from config.h and complete them

These ones are irrelevant to the config but rather to the platform, and
as such are better placed in compiler.h.

Here we take the opportunity for declaring a few extra capabilities:
 - HA_UNALIGNED         : CPU supports unaligned accesses
 - HA_UNALIGNED_LE      : CPU supports unaligned accesses in little endian
 - HA_UNALIGNED_FAST    : CPU supports fast unaligned accesses
 - HA_UNALIGNED_ATOMIC  : CPU supports unaligned accesses in atomics

This will help remove a number of #ifdefs with arch-specific statements.