ARM codegen: Add disassembler, implement return

This program works:

    main() { return 42; }

The disassembler was borrowed from codeflinger, and just modified enough to compile
under C++ without warnings.

Implemented gsym
Implemented a hack verison of li, only works for -256..255
Implemented gjmp
diff --git a/libacc/test b/libacc/test
index 8bf33c8..0b767da 100755
--- a/libacc/test
+++ b/libacc/test
@@ -1,3 +1,3 @@
 #!/bin/sh
-g++ acc.cpp -g -ldl -o tests/acc && tests/acc tests/otcc.c -a x86 -d tests/otcc.out && diff tests/otcc.out tests/otcc.out-orig
-tests/acc tests/simplest.c
+g++ acc.cpp disassem.cpp -g -ldl -o tests/acc && tests/acc tests/otcc.c -a x86 -d tests/otcc.out && diff tests/otcc.out tests/otcc.out-orig
+tests/acc -S tests/returnval.c