Integrate changes #8328,8329,8330,8331,8332,8341,8343,8377

Integrate changes #8328,8329,8330,8331,8332,8341,8343,8377
from mainline.

UTF-8 cleanup.

Subject: [PATCH perl@8327] strings with \x{..} in the middle are corrupted

"\x{FF}\xFF" was broken.

Tests for #8329 and #8330.

Add a note about EBCDIC versus UTF-8 to a potential problem spot.

IRIX compiler noticed that the bof initialization might be
bypassed by control flow.

Make explicit our assumption that (for now) "\x{80}" produces UTF-8.

p4raw-link: @8332 on //depot/perl: bf1fed83f9a0d78ad078e497a12a4e988adb9031
p4raw-link: @8331 on //depot/perl: 299b089d70d012ab45fb81fc8423448962619a13
p4raw-link: @8330 on //depot/perl: 301d3d20746da77e204598157e568e8c22a220b1
p4raw-link: @8329 on //depot/perl: 65e2654b3cf08ae676040518f84657b8a84698e3
p4raw-link: @8328 on //depot/perl: fd400ab9029cac48f6054d57b59a99ec1c5d5e1e

p4raw-id: //depot/maint-5.6/perl@8574
p4raw-integrated: from //depot/perl@8572 'copy in' t/op/bop.t (@7665..)
p4raw-integrated: from //depot/perl@8343 'edit in' toke.c (@8341..)
p4raw-integrated: from //depot/perl@8332 'merge in' utf8.c (@8323..)
p4raw-integrated: from //depot/perl@8328 'merge in' pp_ctl.c pp_sys.c
	regcomp.c regexec.c (@8289..) pp_hot.c (@8303..) op.c (@8313..)
	doop.c pp.c sv.c (@8323..)