Fix compile time warnings building the binutils with gcc 7.1.1.

Fix compile time warnings building the binutils with gcc 7.1.1.

bfd	* elf32-xtensa.c (elf_xtensa_get_plt_section): Increase length of
	plt_name buffer.
	(elf_xtensa_get_gotplt_section): Increase length of got_name
	buffer.
	* mach-o-arm.c (bfd_mach_o_arm_canonicalize_one_reloc): Add a
	default return of FALSE.
	* mach-o-i386.c (bfd_mach_o_i386_canonicalize_one_reloc): Add a
	default return of FALSE.

binutils * dwarf.c (dwarf_vmatoa_1): Do not pass a NULL string pointer to
	sprintf.
	* srconv.c (walk_tree_type): Initialise the spare field of the
	IT_dty structure.

gas	* config/tc-pru.c (md_assemble): Add continue statement after
	handling 'E' operand character.
	* config/tc-v850.c (md_assemble): Initialise the 'insn' variable.