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 openzfs#13193
Closes openzfs#13225

config/always-arch.m4

@@ -29,13 +29,13 @@ AC_DEFUN([ZFS_AC_CONFIG_ALWAYS_ARCH], [
 	sparc64)
 		TARGET_CPU=sparc64
 		;;
+	*)
+		TARGET_CPU=$target_cpu
+		;;
 	esac
 
 	AC_SUBST(TARGET_CPU)
 
-	AM_CONDITIONAL([TARGET_CPU_I386], test $TARGET_CPU = i386)
 	AM_CONDITIONAL([TARGET_CPU_X86_64], test $TARGET_CPU = x86_64)
 	AM_CONDITIONAL([TARGET_CPU_POWERPC], test $TARGET_CPU = powerpc)
-	AM_CONDITIONAL([TARGET_CPU_AARCH64], test $TARGET_CPU = aarch64)
-	AM_CONDITIONAL([TARGET_CPU_SPARC64], test $TARGET_CPU = sparc64)
 ])