[PATCH][Binutils][AArch64] Fix LD crash on weak and undefined TLS symbols. (PR/24602).

[Tamar Christina]

Hi All,

This patch fixes a few linker crashes due to TLS code reaching an assert when it
shouldn't.

The first scenario is with weak TLS symbols that remain weak during linking.  In
this case the mid-end would not have seen a TLS symbol and so wouldn't have
allocated the TLS section.  We currently assert here and the linker crashes with
a not very useful message.

This patch changes this to return the value 0 for the TLS symbol in question
emulating what lld and gold and other BFD targets do.  However because weak TLS
is implementation defined and we don't define any behavior for it I also emit a
warning to the user to inform them of such.

Secondly when a strong TLS reference is undefined. The linker crashes even after
it correctly reported that there is an undefined reference.  This changes it so
that it gracefully exits and reports a useful error.

build on native hardware and regtested on
  aarch64-none-elf, aarch64-none-elf (32 bit host),
  aarch64-none-linux-gnu, aarch64-none-linux-gnu (32 bit host)

Cross-compiled and regtested on
  aarch64-none-linux-gnu, aarch64_be-none-linux-gnu

and no issues.

Ok for master? and for backport to binutils-2.32 after some stew?

Thanks,
Tamar

bfd/ChangeLog:

2019-08-19  Tamar Christina  <tamar.christina@arm.com>

	PR ld/24601
	* elfnn-aarch64.c (aarch64_relocate): Handle weak TLS and undefined TLS.
	Also Pass input_bfd to _bfd_aarch64_elf_resolve_relocation.
	* elfxx-aarch64.c (_bfd_aarch64_elf_resolve_relocation): Use it.
	* elfxx-aarch64.h (_bfd_aarch64_elf_resolve_relocation): Emit warning
	for weak TLS.

ld/ChangeLog:

2019-08-19  Tamar Christina  <tamar.christina@arm.com>

	PR ld/24601
	* testsuite/ld-aarch64/aarch64-elf.exp (undef-tls, weak-tls): New.
	* testsuite/ld-aarch64/undef-tls.d: New test.
	* testsuite/ld-aarch64/undef-tls.s: New test.
	* testsuite/ld-aarch64/weak-tls.d: New test.
	* testsuite/ld-aarch64/weak-tls.s: New test.

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