libdwfl: Add dwfl_module_getsymtab_first_global.

libdwfl: Add dwfl_module_getsymtab_first_global.

New function that provides the index after the last non-local symbol as
returned by dwfl_module_getsym and dwfl_module_getsym_info. Allows users to
first search through all global symbols before searching the local symbols
in the table like dwfl_module_addrsym and dwfl_module_addrsym_info do as
optimization.

Signed-off-by: Mark Wielaard <mjw@redhat.com>