bfd: fix recognition of arch-specific augmentations

This patch fixes _bfd_elf_parse_eh_frame so it will not recognize
machine/architecture specific augmentation characters in EH Frame
CFIs.

Regtested in x86_64-linux-gnu and aarch64-linux-gnu.

bfd/ChangeLog:

	* elf-eh-frame.c (_bfd_elf_parse_eh_frame): Recognize augmentation
	'B' only if targetting aarch64.