Add support for cache descriptor 0x76

This corresponds to an ITLB for 2M/4M pages with 8 entries.
Add support for decoding cache information in cpuid.4

When cpuid.2 reports a cache descriptor of 0xff, that means cpuid.2
doesn't contain any information about caches and that instead you are
supposed to find this information in cpuid.4.

This patch adds support for decoding that information.