BFD: Prevent writing the MIPS _gp_disp symbol into symbol tables

BFD: Prevent writing the MIPS _gp_disp symbol into symbol tables

The _gp_disp is a magic symbol, always implicitly defined by the linker.
It does not make a sense to write it into symbol tables for output files.
Moreover, now if the linker gets a version script, the _gp_disp symbol
gets zero version definition index.  The zero index means[1]:

"The symbol is local, not available outside the object."

But the _gp_disp symbol has GLOBAL binding.  That confuses some tools
like for example the LLD linker when they get such files as inputs.

This patch fixes the problem - it prevents writing the _gp_disp symbol
in regular and dynamic symbol tables.

This was tested by running LD test suite on a mipsel-linux board.

References:

[1] "Linux Standard Base Specification", Section "10.7.2 Symbol
    Version Table", p. 32

2018-05-03  Simon Atanasyan  <simon@atanasyan.com>

bfd/

	* elf32-mips.c: (elf32_mips_fixup_symbol): New function.
	(elf_backend_fixup_symbol): New macro.
	* elfxx-mips.c: (mips_elf_output_extsym): Discard _gp_disp
	handling.
	(_bfd_mips_elf_finish_dynamic_symbol): Likewise.

ld/

	* testsuite/ld-mips-elf/gp-disp-sym.d: New test.
	* testsuite/ld-mips-elf/gp-disp-sym.s: New test source.
	* testsuite/ld-mips-elf/mips-elf.exp: Run the new test.
	* testsuite/ld-mips-elf/mips16-pic-2.ad: Update for _gp_disp
	symbol removal.
	* testsuite/ld-mips-elf/mips16-pic-2.nd: Likewise.
	* testsuite/ld-mips-elf/pic-and-nonpic-3a.dd: Likewise.
	* testsuite/ld-mips-elf/tlslib-o32-hidden.got: Likewise.
	* testsuite/ld-mips-elf/tlslib-o32-ver.got: Likewise.
	* testsuite/ld-mips-elf/tlslib-o32.got: Likewise.