_autovivification: Perl_ck_null is not exported, use PL_check[0] instead

Detected on Windows

Fixed Conflicts:
	C.xs
t/testplc.sh, t/testc.sh: fix test 4 for 5.18
CC: -faelem fix rmg logic for autovivify check

put 2 numeric ops into 2 seperate lines to see which line failed.
add -Dp debuggung output for aelem
harmonize wrongly spelled vivify varnames
-faelem with -fno-autovivify is not stable yet