Added support in gas for mlittle-endian and mbig-endian flags as options.

Added support in gas for mlittle-endian and mbig-endian flags as options.

Updated show usage for MicroBlaze specific assembler options
to include new entries.

This patch has been tested for years of AMD Xilinx Yocto
releases as part of the following patch set:

https://github.com/Xilinx/meta-xilinx/tree/master/meta-microblaze/recipes-devtools/binutils/binutils

Signed-off-by: nagaraju <nagaraju.mekala@amd.com>
Signed-off-by: Neal Frager <neal.frager@amd.com>
---
V1->V2:
 - removed new options which were unnecessary
 - added documentation for MicroBlaze specific options

Signed-off-by: Michael J. Eager <eager@eagercon.com>