sparc64: Add HAVE_FUNCTION_TRACE_MCOUNT_TEST and tidy up.

Check function_trace_stop at ftrace_caller

Toss mcount_call and dummy call of ftrace_stub, unnecessary.

Document problems we'll have if the final kernel image link
ever turns on relaxation.

Properly size 'ftrace_call' so it looks right when inspecting
instructions under gdb et al.

Signed-off-by: David S. Miller <davem@davemloft.net>