Compile exceptions.cc with -fno-omit-frame-pointer on x86

Compile exceptions.cc with -fno-omit-frame-pointer on x86

Selectively using -fomit-frame-pointer when -O is used doesn't make sense
anymore, apparently since gcc 4.6, -O implies -fomit-frame-pointer.

exceptions.cc must be compiled with -fno-omit-frame-pointer on x86, as it uses
RtlCaptureContext, which requires a frame pointer.

	* Makefile.in : Remove setting -fomit-frame-pointer for compiling
	various files, it is already the default.  Set
	-fno-omit-frame-pointer for exceptions.cc on x86.

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>