ld: do not produce one empty output .ctf section for every input .ctf

ld: do not produce one empty output .ctf section for every input .ctf

The trick we use to prevent ld doing as it does for almost all other
sections and copying the input CTF section into the output has recently
broken, causing output to be produced with a valid CTF section followed
by massive numbers of CTF sections, one per .ctf in the input (minus
one, for the one that was filled out by ctf_link).  Their size is being
forcibly set to zero, but they're still present, wasting space and
looking ridiculous.

This is not right:

ld/ld-new  :
section                     size      addr
.interp                       28   4194984
[...]
.bss                       21840   6788544
.comment                      92         0
.ctf                       87242         0
.ctf                           0         0
.ctf                           0         0
[snip 131 more empty sections]
.gnu.build.attributes       7704   6818576
.debug_aranges              6592         0
.debug_info              4488859         0
.debug_abbrev             150099         0
.debug_line               796759         0
.debug_str                237926         0
.debug_loc               2247302         0
.debug_ranges             237920         0
Total                   10865285

The fix is to exclude these unwanted input sections from being present
in the output.  We tried this before and it broke things, because if you
exclude all the .ctf sections there isn't going to be one in the output
so there is nowhere to put the deduplicated CTF. The solution to that is
really simple: set SEC_EXCLUDE on *all but one* CTF section.  We don't
care which one (they're all the same once their size has been zeroed),
so just pick the first we see.

ld/
	* ldlang.c (ldlang_open_ctf): Set SEC_EXCLUDE on all but the
	first input .ctf section.