Add a test case for fast tracepoints' locking mechanism

Add a test case for fast tracepoints' locking mechanism

When installing a fast tracepoint, we create a jump pad with a
spin-lock.  This way, only one thread can collect a given tracepoint at
any time.  This test case checks that this lock actually works as
expected.

This test works by creating a function which overrides the in-process
agent library's gdb_collect function.  On start up, GDBserver will ask
GDB with the 'qSymbol' packet about symbols present in the inferior.
GDB will reply with the gdb_agent_gdb_collect function from the test
case instead of the one from the agent.

gdb/testsuite/ChangeLog:

	* gdb.trace/ftrace-lock.c: New file.
	* gdb.trace/ftrace-lock.exp: New file.