[ARM] Store exception handling information per-bfd instead of per-objfile

[ARM] Store exception handling information per-bfd instead of per-objfile

Based on feedback from Tromey, update the use of objfile_key in gdb/arm-tdep.c
to use bfd_key instead. That way we don't have to re-create the exception
handling data all over again if it was done before for the same BFD.

gdb/ChangeLog:

2019-10-31  Luis Machado  <luis.machado@linaro.org>

	* arm-tdep.c (arm_exidx_data_key): Use bfd_key instead of
	objfile_key.
	(arm_exidx_new_objfile): Adjust to use objfile->obfd instead of
	objfile to fetch per-bfd data.
	(arm_find_exidx_entry): Likewise.

Change-Id: Ia7b3208ea8d788414600fa6d770ac76db0562859