Add stack high register and stack low register for MicroBlaze

Add stack high register and stack low register for MicroBlaze
hardware assisted stack protection, stores stack low / stack high limits
for detecting stack overflow / underflow

binutils/opcodes
          * microblaze-opcm.h: Add REG_SLR_MASK, REG_SHR_MASK, REG_SHR and REG_SLR
          * microblaze-dis.c (get_field_special): Handle REG_SLR_MASK and REG_SHR_MASK
binutils/gas
          * config/tc-microblaze.c (parse_reg): Parse REG_SLR, REG_SHR
binutils/gas
          * gas/microblaze/allinsn.s: Test use of SHR, SLR
          * gas/microblaze/allinsn.d: Likewise