Zero initialise dynamic sections before use.

Due to the mechanism that is currently used by ARC in order to size the
dynamic sections (specifically, they are sized during
elf_arc_check_relocs, then filled later) when garbage collection kicks
in, the tail end of the dynamic sections can be left unused.

This patch ensures that any unused space will be filled with zero, which
corresponds to the R_ARC_NONE relocation type.

bfd/ChangeLog:

	* elf32-arc.c (elf_arc_size_dynamic_sections): Zero initialise
	contents of dynamic sections before use.