elf32_arm_get_synthetic_symtab memory leak

ARM get_synthetic_symtab reads .plt and caches that data.  Caching the
data doesn't make a lot of sense since get_synthetic_symtab is only
called once per bfd, and the memory might be put to better use.  It
also leaks on closing the bfd.

	* elf32-arm.c (elf32_arm_get_synthetic_symtab): Don't cache
	plt contents.  Free plt data before returning.