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/powerpc/assembler.cpp

@@ -8,7 +8,7 @@
    There is NO WARRANTY for this software.  See license.txt for
    details. */
 
-#include "assembler.h"
+#include "codegen/assembler.h"
 #include "alloc-vector.h"
 
 #define CAST1(x) reinterpret_cast<UnaryOperationType>(x)
@@ -2446,6 +2446,8 @@ class MyArchitecture: public Assembler::Architecture {
     }
   }
 
+  virtual Assembler* makeAssembler(Allocator* allocator, Zone* zone);
+
   virtual void acquire() {
     ++ referenceCount;
   }
@@ -2858,23 +2860,20 @@ class MyAssembler: public Assembler {
   MyArchitecture* arch_;
 };
 
+Assembler* MyArchitecture::makeAssembler(Allocator* allocator, Zone* zone) {
+  return new(zone) MyAssembler(this->c.s, allocator, zone, this);
+}
+
 } // namespace
 
-namespace vm {
+namespace avian {
+namespace codegen {
 
 Assembler::Architecture*
-makeArchitecture(System* system, bool)
+makeArchitecturePowerpc(System* system, bool)
 {
   return new (allocate(system, sizeof(MyArchitecture))) MyArchitecture(system);
 }
 
-Assembler*
-makeAssembler(System* system, Allocator* allocator, Zone* zone,
-              Assembler::Architecture* architecture)
-{
-  return
-    new(zone) MyAssembler(system, allocator, zone,
-                          static_cast<MyArchitecture*>(architecture));
-}
-
-} // namespace vm
+} // namespace codegen
+} // namespace avian