x86: Fix for cache computation on AMD legacy cpus.

Some legacy AMD CPUs and hypervisors have the _cpuid_ '0x8000_001D'
set to Zero, thus resulting in zeroed-out computed cache values.
This patch reintroduces the old way of cache computation as a
fail-safe option to handle these exceptions.
Fixed 'level4_cache_size' value through handle_amd().

Reviewed-by: Premachandra Mallappa <premachandra.mallappa@amd.com>
Tested-by: Florian Weimer <fweimer@redhat.com>