misc: Let the compiler choose whether to omit frame pointer

Currently CONFIG_OMIT_FRAME_POINTER is a boolean and defaults to N,
which increases code size. This is intended to improve debugability
but on many architectures the use of this option does not actually
compromise debugging. Further the compiler already knows whether or not
omitting the frame pointer is harmful to debugging and its defaults
are selected accordingly.

By making this choice optional we can have a sane default on *all*
architectures by letting the compiler decide for us.

This patch significantly improves the default code generation on
arm (thumb), nios2 and arc. The benefit looks to be about between 5%
and 10% code size reduction depending upon architecture). x86 is
unaffected.

Change-Id: I5790634a40e2462cc1089dce4087040833793ae7
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>