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

v2 addresses issues found as well as review feedback; see individual
patches for details. It (patch 2 in particular) raises several new
questions as well, though.

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

Jan