Integrate changes #8267,8272[perlio],8274,8298,8300,8303,

Integrate changes #8267,8272[perlio],8274,8298,8300,8303,
8305,8323,8324 from mainline.  The 8267,8272, and 8298 were
not really integrated but instead salvaged by hand
(they had too many dependencies on the development release
to be cleanly integratable).

Subject: more UTF8 test suites and an UTF8 patch

Tweak for MULTIPLICITY/USE_PERLIO

Signedness nit.

Turn SvUTF8 off if not required in pp_chr and pp_stringify.

Use the UTF8_XXX macros in is_utf8_char().

Rewrite pp_concat() in terms of sv_catsv().  The . operator
should now be UTF-8-proof.

Subject: [PATCH perl@8269] scanning two hex-constants
fails on EBCDIC environment (script length.t)

Add some Unicode chop() tests.

p4raw-link: @8272 on //depot/perlio: 7948272db9c63907ea8e92fdd3436cdaab2f9cce
p4raw-link: @8267 on //depot/perl: 1aa99e6b6d14c469ac825dde483d9c9f913a3ee2

p4raw-id: //depot/maint-5.6/perl@8572
p4raw-integrated: from //depot/perl@8571 'copy in' t/op/chop.t
	(@3319..) 'merge in' utf8.h (@8289..)
p4raw-edited: from //depot/perl@8323 'edit in' doop.c (@8289..) pp.c
	(@8299..) utf8.c (@8300..) toke.c (@8306..)
p4raw-integrated: from //depot/perl@8323 'merge in' sv.c (@8301..)
p4raw-edited: from //depot/perl@8303 'edit in' pp_hot.c (@8299..)