Added new instruction support

Added two new instructions, wdc.ext.clear and wdc.ext.flush, to enable MicroBlaze to flush an external cache,
which is used with the new coherency support for multiprocessing

Signed-off-by:nagaraju <nmekala@xilix.com>

Signed-off-by: David Holsgrove <david.holsgrove@xilinx.com>
Added mlittle-endian and mbig-endian flags

Added support in the binutils to accept mlittle-endian and mbig-endian flags by the assembler.
Added MicroBlaze specific assembler options in help with the new flags

Signed-off-by:nagaraju <nmekala@xilix.com>

Signed-off-by: David Holsgrove <david.holsgrove@xilinx.com>