Add new insns: LCALL and ICALL

After more than a year, the S16X4 CPU finally gets the ability to call subroutines!
Introduce ABORT_I signal to support external MMUs.
Include S16X4A-optimized assembler.
Merge pull request #51 from sam-falvo/s16x4a

S16x4a
Merge branch 'master' into sts-1