op.c:ck_rvconst: Allocate GV pad slots like constants

op.c:ck_rvconst: Allocate GV pad slots like constants

Requesting a pad slot for a constant (and GVs in pads are like con-
stants, in that the pad slot will always hold that same value, which
is shared in recursion) will instruct pad_alloc to avoid anything
marked PADTMP; i.e., a  slot used as a target by a previous statement.

Without this, we get an assertion failure from ‘miniperl -MExporter’
under -Accflags=-DUSE_BROKEN_PAD_RESET and threads.

With this commit, we get ‘Modification of a read-only value attempted’
instead, which is some progress. :-)