Make bfd_get_8 return a bfd_vma like other bfd_get_* functions

And similarly for bfd_get_signed_8.

bfd/
	* libbfd.c (bfd_get_8): Return a bfd_vma.
	(bfd_get_signed_8): Return a bfd_signed_vma.
	* bfd-in2.h: Regenerate.
binutils/
	* od-xcoff.c (dump_dumpx_core): Adjust for bfd_h_get_8 change.