Use string_to_event_location_basic in guile.

This patch, analogous to the previous python patch, implements proper
legacy linespec support in guile code using the newly introduced
string_to_event_location_basic.

gdb/ChangeLog

	* guile/scm-breakpoint.c (gdbscm_register_breakpoint_x): Skip
	leading whitespace and use string_to_event_location_basic instead
	of new_linespec_location.

gdb/testsuite/ChangeLog

	* gdb.guile/scm-breakpoint.exp (test_bkpt_address): New procedure.
	(toplevel): Call test_bkpt_address.