microblaze: Fix ASM optimized code for LE

Microblaze little-endian doesn't support ASM optimized library
functions(memcpy/memmove). Kconfig doens't contain
any information about endian that's why it is necessary to
check it in the source code.
The code is used with barrel shifter is used.

Signed-off-by: Michal Simek <monstr@monstr.eu>