config: always-arch: prune unused TARGET_CPU_*, add TARGET_CPU catch-all

This fixes (harmless) error spew from configuring on, e.g., armv6l

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13193
Closes #13225