make PL_reg_curpm global

make PL_reg_curpm global

Currently PL_reg_curpm is actually #deffed to a field within PL_reg_state;
promote it into a fully autonomous perl-interpreter variable.

PL_reg_curpm points to a fake PMOP that's used to temporarily point
PL_curpm to, that we can hang the current regex off, so that this works:

    "a" =~ /^(.)(?{ print $1 })/ # prints 'a'

It turns out that it doesn't need to be saved and restored when we
recursively enter the regex engine; that is already handled by saving and
restoring which regex is currently attached to PL_reg_curpm.
So we just need a single global (per interpreter) placeholder.

Since we're shortly going to get rid of PL_reg_state, we need to move it
out of that struct.