Merge 141243f from http://github.com/eloaders/libcpuid

This adds support for x2apic detection. This is not a direct merge, since

- the feature is spelled as 'x2apic' instead of 'x2APIC', for consistency with other flags;
- tests are added
- the id of the feature is moved to the end of the cpu_feature_t enum, for binary
  compatibility.