Add process.plt to probe plt entries.

Add process.plt to probe plt entries.

	* dwflpp.cxx (iterate_over_plt):  New to callback through plt entries.
	* dwflpp.h: Likewise.
	* tapsets.cxx (TOK_PLT): New.
	(base_query::query_plt_callback, query_plt):  New methods.
	(base_query):  New members has_plt, has_statement, plt_val.
	(base_query::base_query):  Set new members.
	(dwarf_query::query_module_symtab):  Use raw address for plt entries.
	(dwarf_query::add_probe_point): Don't reloc the plt entry address.
	(query_module):  Call iterate_over_plt.
	(plt_expanding_visitor):  New to expand $$name.
	(dwarf_derived_probe::register_patterns):  Add new probe tags.