allow codegen targets (Architectures and Assemblers) to co-exist

The primary motivation behind this is to allow all the different Assemblers
to be built at once, on a single machine. This should dramatically reduce
the time required to make sure that a particular change doesn't break
the build for one of the not-so-common architectures (arm, powerpc)

Simply pass "codegen-targets=all" to make to compile all
src/codegen/<arch>/assembler.cpp.

Note that while these architectures are built, they will not be fully-
functional.  Certain stuff is assumed to be the same across the entire
build (such as TargetBytesPerWord), but this isn't the case anymore.

src/codegen/targets.cpp

@@ -0,0 +1,37 @@
+/* Copyright (c) 2008-2012, Avian Contributors
+
+   Permission to use, copy, modify, and/or distribute this software
+   for any purpose with or without fee is hereby granted, provided
+   that the above copyright notice and this permission notice appear
+   in all copies.
+
+   There is NO WARRANTY for this software.  See license.txt for
+   details. */
+
+#include "codegen/targets.h"
+#include "environment.h"
+
+namespace avian {
+namespace codegen {
+
+vm::Assembler::Architecture* makeArchitectureNative(vm::System* system, bool useNativeFeatures UNUSED) {
+#ifndef AVIAN_TARGET_ARCH
+  #error "Must specify native target!"
+#endif
+
+#if AVIAN_TARGET_ARCH == AVIAN_ARCH_UNKNOWN
+  system->abort();
+  return 0;
+#elif (AVIAN_TARGET_ARCH == AVIAN_ARCH_X86) || (AVIAN_TARGET_ARCH == AVIAN_ARCH_X86_64)
+  return makeArchitectureX86(system, useNativeFeatures);
+#elif AVIAN_TARGET_ARCH == AVIAN_ARCH_ARM
+  return makeArchitectureArm(system, useNativeFeatures);
+#elif AVIAN_TARGET_ARCH == AVIAN_ARCH_POWERPC
+  return makeArchitecturePowerpc(system, useNativeFeatures);
+#else
+  #error "Unsupported codegen target"
+#endif
+}
+
+} // namespace codegen
+} // namespace avian