don't share per-BFD data if relocations are needed

don't share per-BFD data if relocations are needed

Right now we always share per-BFD data across objfiles, if there is a
BFD.  This works fine.  However, we're going to start sharing more
data, and sometimes this data will come directly from sections of the
BFD.  If such a section has SEC_RELOC set, then the data coming from
that section will not be truly sharable -- the section will be
program-space-dependent, and re-read by gdb for each objfile.

This patch disallows per-BFD sharing in this case.  This is a bit
"heavy" in that we could in theory examine each bit of shared data for
suitability.  However, that is more complicated, and SEC_RELOC is rare
enough that I think we needn't bother.

Note that the "no sharing" case is equivalent to "gdb works as it
historically did".  That is, the sharing is a new(-ish) optimization.

Built and regtested on x86-64 Fedora 18.

	* gdb_bfd.c (struct gdb_bfd_data) <relocation_computed,
	needs_relocations>: New fields.
	(gdb_bfd_requires_relocations): New function.
	* gdb_bfd.h (gdb_bfd_requires_relocations): Declare.
	* objfiles.c (get_objfile_bfd_data): Disallow sharing if
	the BFD needs relocations applied.