move probes to be per-bfd

move probes to be per-bfd

This patch moves the probe data from the objfile to the per-BFD
object.  This lets the probes be shared between different inferiors
(and different objfiles when dlmopen is in use, should gdb ever handle
that).

2014-03-03  Tom Tromey  <tromey@redhat.com>

	* elfread.c (probe_key): Change to bfd_data.
	(elf_get_probes, probe_key_free, _initialize_elfread): Probes are
	now per-BFD, not per-objfile.
	* stap-probe.c (stap_probe_destroy): Update comment.
	(handle_stap_probe): Allocate on the per-BFD obstack.