microMIPS: Add SYSCALL instruction decoding

microMIPS: Add SYSCALL instruction decoding

Fix a bug with commit 4cc0665f24bb ("microMIPS support"),
<https://sourceware.org/ml/gdb-patches/2012-05/msg00724.html>, and add
missing microMIPS SYSCALL instruction decoding needed to determine the
location to put a breakpoint at when single-stepping though a syscall.

	gdb/
	* mips-tdep.c (micromips_next_pc): Add SYSCALL instruction
	decoding.