[PATCH][gold] Fix offset calculation when applying AArch64 TLSDESC relocations.

[Cary Coutant]

> If a custom linker script with an unexpected relative layout of .got
> and .got.plt sections was used, gold might produce a wrong offset
> when applying R_AARCH64_TLSDESC_* relocations.
>
> This patch fixes the issue by calculating "got_tlsdesc_offset"
> in a more direct way.
>
> Best regards,
> Igor Kudrin
> C++ Developer, Access Softek, Inc.
>
> ---
> gold/ChangeLog
>
>         * aarch64.cc (Target_aarch64::Relocate::relocate_tls):
>         Make got_tlsdesc_offset signed and fix its calculation.
>         * testsuite/Makefile.am (aarch64_tlsdesc): New test.
>         * testsuite/Makefile.in: Regenerate.
>         * testsuite/aarch64_tlsdesc.s: New test source file.
>         * testsuite/aarch64_tlsdesc.sh: New test script.
>         * testsuite/aarch64_tlsdesc.t: New test linker script.

This is OK. Thanks!

-cary