fix threaded NO_TAINT_SUPPORT build errors

NO_TAINT_SUPPORT incorrectly called Perl_croak without aTHX_.

To fix this, change Perl_croak to Perl_croak_nocontext, since it is
slightly more efficient in instruction size than Perl_croak.
silence warnings under NO_TAINT_SUPPORT

The are lots of places where local vars aren't used when compiled
with NO_TAINT_SUPPORT.