x86, cacheinfo: Enable L3 CID only on AMD

Final stage linking can fail with

 arch/x86/built-in.o: In function `store_cache_disable':
 intel_cacheinfo.c:(.text+0xc509): undefined reference to `amd_get_nb_id'
 arch/x86/built-in.o: In function `show_cache_disable':
 intel_cacheinfo.c:(.text+0xc7d3): undefined reference to `amd_get_nb_id'

when CONFIG_CPU_SUP_AMD is not enabled because the amd_get_nb_id
helper is defined in AMD-specific code but also used in generic code
(intel_cacheinfo.c). Reorganize the L3 cache index disable code under
CONFIG_CPU_SUP_AMD since it is AMD-only anyway.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>
LKML-Reference: <20100218184210.GF20473@aftab>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>