gdbserver/s390: Switch on tracepoint support.

gdbserver/s390: Switch on tracepoint support.

Also adds s390 support to gdb.trace testsuite.

gdb/gdbserver/ChangeLog:

	* linux-s390-low.c (s390_supports_tracepoints): New function.
	(struct linux_target_ops): Fill supports_tracepoints hook.

gdb/testsuite/ChangeLog:

	* gdb.trace/ftrace.exp: Set arg0exp for s390.
	* gdb.trace/mi-trace-frame-collected.exp: Expect 4 registers on s390.
	* gdb.trace/mi-trace-unavailable.exp: Set pcnum for s390, add gpr0num
	variable for GPR 0 instead of assuming it is register 0.
	* gdb.trace/trace-common.h: Add s390 fast tracepoint placeholder.
	* lib/trace-support.exp: Add s390 registers.