As discussed in Prague, with some minor add-ons (patch 2 replacing a much
older patch under the same title).

v3 addresses issues found as well as review feedback; see individual
patches for details. Patch 3 in particular continues to raises several
questions as well, though.

1: build: make cc-option properly deal with unrecognized sub-options
2: build: permit Kconfig control over how to deal with unsatisfiable choices
3: allow Kconfig control over psABI level
4: use POPCNT for hweight<N>() when available
5: short-circuit certain cpu_has_* when x86-64-v{2,3} are in effect

Jan