asan: buffer overflow in elf32_arm_get_synthetic_symtab

Guard against fuzzed files where .plt size isn't commensurate with
plt relocations.

	* elf32-arm.c (elf32_arm_plt0_size): Add data_size param.
	Return -1 if data_size is too small.
	(elf32_arm_plt_size): Likewise.  Delete temp var.  Formatting.
	(elf32_arm_get_synthetic_symtab): Adjust to suit.