MIPS: Tracing: Add static function tracer support for MIPS

If -pg of gcc is enabled with CONFIG_FUNCTION_TRACER=y. a calling to
_mcount will be inserted into each kernel function. so, there is a
possibility to trace the kernel functions in _mcount.

This patch add the MIPS specific _mcount support for static function
tracing. by default, ftrace_trace_function is initialized as
ftrace_stub(an empty function), so, the default _mcount will introduce
very little overhead. after enabling ftrace in user-space, it will jump
to a real tracing function and do static function tracing for us.

and -ffunction-sections is incompatible with -pg, so, disable it when
ftracer is enabled.

Signed-off-by: Wu Zhangjin <[email protected]>
Reviewed-by: Steven Rostedt <[email protected]>
Cc: Nicholas Mc Guire <[email protected]>
Cc: [email protected]
Cc: Wu Zhangjin <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: [email protected]
Cc: [email protected]
Patchwork: http://patchwork.linux-mips.org/patch/672/
Signed-off-by: Ralf Baechle <[email protected]>

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