Factor out the common code in lookup_{static,global}_symbol

Factor out the common code in lookup_{static,global}_symbol

The two functions are extremely similar; this factors out their code into
a shared _internal function.

gdb/ChangeLog:

2019-08-26  Christian Biesinger  <cbiesinger@google.com>

	* symtab.c (lookup_static_symbol): Call the new function (and move
	it down to be next to lookup_global_symbol).
	(struct global_sym_lookup_data): Add block_enum member and rename to...
	(struct global_or_static_sym_lookup_data): ...this.
	(lookup_symbol_global_iterator_cb): Pass block_index instead of
	GLOBAL_BLOCK to lookup_symbol_in_objfile and rename to...
	(lookup_symbol_global_or_static_iterator_cb): ...this.
	(lookup_global_or_static_symbol): New function.
	(lookup_global_symbol): Call new function.