Replace solib_global_lookup with gdbarch_iterate_over_objfiles_in_search_order

Replace solib_global_lookup with gdbarch_iterate_over_objfiles_in_search_order

All implementations of either function use it for the same purpose (except
Darwin, which is a no-op): to prefer a symbol in the current objfile over
symbols with the same name in other objfiles. There does not seem to be a
reason to have both mechanisms for that purpose.

gdb/ChangeLog:

2019-09-20  Christian Biesinger  <cbiesinger@google.com>

	* solib-darwin.c (darwin_lookup_lib_symbol): Remove.
	(_initialize_darwin_solib): Don't set
	darwin_so_ops.lookup_lib_global_symbol.
	* solib-svr4.c (set_solib_svr4_fetch_link_map_offsets): Call
	set_gdbarch_iterate_over_objfiles_in_search_order.
	(elf_lookup_lib_symbol): Rename to...
	(svr4_iterate_over_objfiles_in_search_order): this, and update
	to iterate semantics.
	(_initialize_svr4_solib): Don't set lookup_lib_global_symbol.
	* solib.c (solib_global_lookup): Remove.
	* solist.h (struct target_so_ops): Remove lookup_lib_global_symbol.
	(solib_global_lookup): Remove.
	* symtab.c (lookup_global_or_static_symbol): Remove call to
	solib_global_lookup.