[kernel][x86] Implement startup-time code patching for SMAP instructions

Add a macro, APPLY_CODE_PATCH_FUNC(), which allocates some placeholder
instructions and declares how to replace them with working code on
kernel startup.

The initial use case is filling out the STAC and CLAC instructions
conditionally depending on whether the CPU supports SMAP.

The mechanism is a little more general than this initial use case
requires. My intention is the mechanism will be usable for inline
assembly that can get inlined into multiple places.

Change-Id: I213504db574f58c2d093d2648771c03048775c09