sim: pru: Add support for LMBD instruction

Binutils support for LMBD instruction was merged [1]. So add it also
to simulator.

LMBD instruction does left-most-bit-detection. It returns 32 if
the given bit value is not found in the provided word value.

[1] https://sourceware.org/pipermail/binutils/2020-October/113901.html

sim/pru/ChangeLog:

	* pru.h (RS1SEL): New macro.
	(RS1_WIDTH): New macro.
	* pru.isa: Describe the LMBD instruction.

sim/testsuite/sim/pru/ChangeLog:

	* lmbd.s: New test.