ftrace: Be more specific about arch impact when function tracer is enabled

It was brought up that on ARMv7, that because the FUNCTION_TRACER does not
use nops to keep function tracing disabled because of the use of a link
register, it does have some performance impact.

The start of functions when -pg is used to compile the kernel is:

	push    {lr}
	bl      8010e7c0 <__gnu_mcount_nc>

When function tracing is tuned off, it becomes:

	push    {lr}
	add   sp, sp, #4

Which just puts the stack back to its normal location. But these two
instructions at the start of every function does incur some overhead.

Be more honest in the Kconfig FUNCTION_TRACER description and specify that
the overhead being in the noise was x86 specific, but other architectures
may vary.

Link: https://lore.kernel.org/all/[email protected]/
Link: https://lkml.kernel.org/r/[email protected]

Reported-by: Sascha Hauer <[email protected]>
Acked-by: Sascha Hauer <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: rostedt

kernel/trace/Kconfig

@@ -194,7 +194,8 @@ config FUNCTION_TRACER
 	  sequence is then dynamically patched into a tracer call when
 	  tracing is enabled by the administrator. If it's runtime disabled
 	  (the bootup default), then the overhead of the instructions is very
-	  small and not measurable even in micro-benchmarks.
+	  small and not measurable even in micro-benchmarks (at least on
+	  x86, but may have impact on other architectures).
 
 config FUNCTION_GRAPH_TRACER
 	bool "Kernel Function Graph Tracer"