Add NaCl build scripts
NaCl GC improvements

- inline managed code implementation
  (add x86 test mem imm8 codegen macro for this as well)
- clean up libgc NaCl code
- centralize mono_nacl_gc into mini.c
NaCl amd64 codegen fixes

- mostly more code fixes to deal with a 64-bit ILP32 machine
NaCl x86 codegen fixes
Add explicit laddcc/lsubcc IR opcodes

This allows ladd/lsub to decompose into CC instructions
that correctly maintain the width of the operands.  This
was a problem on NaCl amd64 specifically which has 32-bit
longs and ints, but 8 byte registers.
Test for divide by zero for NaCl in software