dump.c: Don’t dump GvFLAGS as part of GP

The flags are not actually stored in the GP.  Dumping them as part of
it implies that they are shared between globs that share the same GP,
which is not the case.
Peek.t needs to know about the prev. change
Fix assertion failure with undef &my_sub/&anon

$ ./perl -Ilib -le 'use experimental lexical_subs; my sub x; undef &x;'
Assertion failed: (isGV_with_GP(_gvname_hek)), function Perl_leave_scope, file scope.c, line 1035.
Abort trap: 6

pp_undef undefines a subroutine via cv_undef, which wipes out the
name, and then restores the name again afterwards.

For subs with GVs, it would call CvGV_set afterwards with the same gv.
But cv_undef could have freed the GV, if the CV held the only refer-
ence count.

I caused this for lexical subs a few commits ago in ae77754ae (because
CvGV will always return non-null; in fact the CvNAME_HEK code in
pp_undef is no longer exercised, but I will address that soon).

For anonymous subs it is older:

$ perl5.14.4 -e '$_ = sub{}; delete $::{__ANON__}; undef &$_; use Devel::Peek; Dump $_'
SV = IV(0x7fed9982f9c0) at 0x7fed9982f9d0
  REFCNT = 1
  FLAGS = (ROK)
  RV = 0x7fed9982f9e8
  SV = PVCV(0x7fed9982e290) at 0x7fed9982f9e8
    REFCNT = 2
    FLAGS = (PADMY,WEAKOUTSIDE,CVGV_RC)
    COMP_STASH = 0x7fed99806b68	"main"
    ROOT = 0x0
    GVGV::GV = 0x7fed9982fa48Assertion failed: (isGV_with_GP(_gvname_hek)), function Perl_do_gvgv_dump, file dump.c, line 1477.
Abort trap: 6

(Probably commit 803f2748.)

Presumably that could be made to crash in other ways than introspec-
tion, but it is much harder.

This commit fixes the problem by fiddling with reference counts.  But
this is only a temporary fix.  I think I plan to stop cv_undef from
removing the name (gv/hek) when called from pp_undef.