Fix sfio breakage caused by the fix for RT #78494.

Fix sfio breakage caused by the fix for RT #78494.

Commit 2e0cfa16dea85dd3 (Feb 2011) fixed RT #78494, but inadvertently broke
non-PerlIO configurations. -Uuseperlio was restored in March 2011 with commit
b6ae43b72863e12d, but this change is needed to restore the sfio build.

This is the last fix needed to get blead building with -Dusesfio, after 10
years of breakage. However, many many tests still fail (and one hangs).
Note, specifically this is only tested with sfio97. Perl is making a couple
of cheating assumptions which break with sfio2005, and I've not attempted to
fix Perl's naughtiness to get it to build with current sfio. (Nor has anyone
else in the past 8 years, it would seem. Or even reported a problem.)