Turned sim65 into a lightweight cc65 execution environment.

The sim65 source code has been a construction site for over a decade.
I was looking for a simple cc65 program execution environment for
regression tests. So I decided to re-purpose sim65 for that task by
removing about everything but the 6502 emulation.

There's no memory mapped i/o emulation whatsoever. Rather exit(),
open(), close(), read() and write() calls are supported by mapping
them through a thin paravirtualization layer to the corresponding
host os functions.

Note: The sim65 6502 emulation provides means to switch between
6502 and 65C02 emulation but currently there are no actual 65C02
opcodes implemented.

src/Makefile

@@ -9,6 +9,7 @@ PROGS = ar65  \
         grc65 \
         ld65  \
         od65  \
+        sim65 \
         sp65
 
 CA65_INC := $(abspath ../asminc)