Add support for the LMBD (left-most bit detect) instruction to the PRU assembler.

include	* opcode/pru.h: Add LMBD (left-most bit detect) opcode index
opcodes * pru-opc.c: Add opcode description for LMBD (left-most bit detect)
gas 	* testsuite/gas/pru/misc.s: Add tests for lmbd (left-most bit detect)
 	* testsuite/gas/pru/misc.d: Add tests for lmbd (left-most bit