objdump, readelf: Emit "CU:" format only when wide output is requested

objdump, readelf: Emit "CU:" format only when wide output is requested

As pre-approved by Alan in
https://sourceware.org/pipermail/binutils/2021-September/118019.html
and I believe people have run into getting testsuite failures for
test-environments with "long" directory names, at least once more
since that time.  Enough.  I grepped the gas, binutils and ld
testsuites for "CU:" to catch target-specific occurrences, but I
noticed none.  I chose to remove "CU:" on the objdump tests instead of
changing options to get the wide format, so as to keep the name of the
test consistent with actual options; but added it to the readelf
options for the gas test as I believe the "CU:" format is preferable.

Tested for cris-elf and native x86_64-pc-linux-gnu.

binutils:
	* dwarf.c (display_debug_lines_decoded): Don't check the
	string length of the directory, instead emit the "CU: dir/name"
	format only if wide output is requested.
	* testsuite/binutils-all/dw5.W, testsuite/binutils-all/objdump.WL:
	Adjust accordingly.

gas:
	* testsuite/gas/elf/dwarf-5-loc0.d: Add -W to readelf options.