sv.c: Make sv_force_normal always croak on ro SVs

sv.c: Make sv_force_normal always croak on ro SVs

Commit 8990e30710 (perl 5.0 alpha 6) changed several SvREADONLY checks
to apply only to run time.  This is convenient when it comes to modify-
ing op trees, whose SVs are, for the most part, marked read-only.

But several pieces of code that relied on this were buggy when dealing
with constants created by ‘use constant’.  To make it harder to add
such bugs, I have changed all code that relies on this exception and
am now removing it.