bfd/elf64-aarch64.c: Use ELF64_R_SYM instead of ELF32_R_SYM.

Fix incorrect usage of ELF32_R_SYM introduced in ifunc changes.

bfd/ChangeLog:

2013-06-11  Will Newton  <will.newton@linaro.org>

	* elf64-aarch64.c (elf_aarch64_get_local_sym_hash): Use
	ELF64_R_SYM instead of ELF32_R_SYM.