Allow passing a block to lookup_global_symbol_from_objfile

Allow passing a block to lookup_global_symbol_from_objfile

This has no behavior change in itself, but allows a future patch
to add a function to the Python API to look up symbols in the
static block.

gdb/ChangeLog:

2019-07-24  Christian Biesinger  <cbiesinger@google.com>

	* compile/compile-object-load.c (compile_object_load): Pass GLOBAL_SCOPE.
	* solib-spu.c (spu_lookup_lib_symbol): Pass GLOBAL_SCOPE.
	* solib-svr4.c (elf_lookup_lib_symbol): Pass GLOBAL_SCOPE.
	* symtab.c (lookup_global_symbol_from_objfile): Add a scope parameter.
	* symtab.h (lookup_global_symbol_from_objfile): Likewise.