<sys/platform/x86.h>: Add Intel LAM support

Add Intel Linear Address Masking (LAM) support to <sys/platform/x86.h>.
HAS_CPU_FEATURE (LAM) can be used to detect if LAM is enabled in CPU.

LAM modifies the checking that is applied to 64-bit linear addresses,
allowing software to use of the untranslated address bits for metadata.