Emit a warning when -z relro is unsupported

[Alan Modra]

I believe the ia64-linux glibc problem is triggered by a fairly simple
glibc build bug.  There is a linker bug too.

I was able to reproduce the problem, with
.IA_64.unwind__libc_freeres_fn being mapped to bfd_abs_section, which
is what happens when a section is discarded by a linker script.

On looking at glibc/shlib.lds I see toward the end:

  .gnu.attributes 0 : { KEEP (*(.gnu.attributes)) }
  /DISCARD/ : { *(.note.GNU-stack) *(.gnu_debuglink) *(.gnu.lto_/home/local/bin/../lib/gcc/ia64-linux/9.3.1/../../../../ia64-linux/bin/ld: warning: -z relro ignored
*) }
}

Uh oh, the script will discard all orphan sections due to *(*)
appearing.  (Yes there are some odd section match patterns before
that from the warning, but the final * gets everything.)

Now .IA_64.unwind__libc_freeres_fn is not an orphan, but it is
SHF_LINK_ORDER and __libc_freeres_fn is its linked-to section, and
that one is an orphan.  ldelf_before_place_orphans handles linked-to
sections, correctly discarding .IA_64.unwind__libc_freeres_fn but
failing to set SEC_EXCLUDE which would avoid the ldwrite.c assertion.

glibc patch attached

-- 
Alan Modra
Australia Development Lab, IBM
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