This commit adds a GDB workaround for the GDBserver bug exposed by
commit f2ffa92b ("gdb: Eliminate the 'stop_pc' global"), so that
newer GDBs can continue working with older GDBservers.

gdb/ChangeLog:
2018-07-11  Pedro Alves  <palves@redhat.com>

	PR gdb/23377
	* remote.c (remote_target::remote_detach_pid): Call
	set_current_process.