Fix GOT relocation overflow on SPARC.

Fix GOT relocation overflow on SPARC.

There are 2 failures left in the linker testsuite on SPARC64/Linux and
they are caused by 2 different issues leading to the same end effect:
the overflow of the relocation section for the GOT, i.e. the linker
generates too many dynamic relocations for the GOT wrt the size of the
relocation section, leading to memory corruption and missing relocations
in the final binary.

The first issue was introduced by:
  https://sourceware.org/ml/binutils/2017-06/msg00368.html
which makes the linker generate more R_SPARC_RELATIVE relocations for
the GOT without adjusting the size of the relocation section.  This is
fixed by (1) preventively adjusting this size in allocate_dynrelocs and
(2) generating R_SPARC_NONE if needed when R_SPARC_GOTDATA_OP is relaxed.

The second issue is that we generate a GOT relocation for an undefined
weak symbol with non-default visibility in a PIC binary without accounting
for that  in the size of the relocation section.  Since the address of the
symbol should resolve to 0 at run time, it is fixed by not generating the
relocation at all, i.e. leaving the GOT entry zeroed.

bfd/
	* elfxx-sparc.c (UNDEFINED_WEAK_RESOLVED_TO_ZERO): Reorder conditions.
	(sparc_elf_append_rela): Assert that there is enough room in section.
	(_bfd_sparc_elf_copy_indirect_symbol): Fix formatting.
	(_bfd_sparc_elf_adjust_dynamic_symbol): Minor tweak.
	(allocate_dynrelocs): Remove outdated comments and reorder conditions.
	For a symbol subject to a GOT relocation, reserve a slot in the
	relocation section if the symbol isn't dynamic and we are in PIC mode.
	(_bfd_sparc_elf_relocate_section) <R_SPARC_GOTDATA_OP>: If relocation
	is relaxed and a slot was reserved, generate a R_SPARC_NONE relocation.
	<R_SPARC_GOTDATA_OP_HIX22>: Adjust comments.
	<R_SPARC_PC10>: Reorder conditions.  Remove always-false assertion.
	(_bfd_sparc_elf_finish_dynamic_symbol): Rename local_undefweak into
	resolved_to_zero.  Do not generate a dynamic GOT relocation for an
	undefined weak symbol with non-default visibility.  Remove superfluous
	'else' and fix formatting.