sim_core.c: fix implimentation of muls instruction

specification for muls insruction states 2 cycles, not one as implimented.

bug find credit goes to: Shay Green <gblargg@gmail.com>

	modified:   sim_core.c
Merge pull request #30 from bsekisser/sim-core-muls-bugfix

sim_core.c: fix implimentation of muls instruction