nios2: set appropriate compiler flags for math support

Nios II CPUs vary in configuration on whether they support
'mul', 'mulx', and 'div' family of instructions. The compiler
can be told to use GCC integer library routines instead if
needed.

Ideally we would just pull the configuration out of system.h,
but pulling include file #defines into the Make environment
will involve some build system work that is best left to a
later improvement.

We've decided to take this build-time approach rather than
handle unimplemented instruction exceptions, so remove the
hook in exception.S

Change-Id: I05be0d5ed4c1a49b23dca1550ee66fd5891044d2
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>