Remove all TARGET_[ARM|X86|MIPS] #ifdefs

Two steps forward, one step back towards elimination of the
"#include" build model for target-specific compilers.  This CL
does some restructuring to eliminate all of the TARGET_xxx #ifdefs
and convert them to run-time tests.

Considerable work is still required to fully eliminate the multiple
builds.  In particular, much of the "common" codegen code relies on
macros defined by the target-specific [Arm|X86|Mips]Lir.h include file.

Next step is more restructuring to better isolate target-independent
code generation code.

Change-Id: If6efbde65c48031a48423344d8dc3e2ff2c4ad9d
diff --git a/src/compiler/codegen/Ralloc.h b/src/compiler/codegen/Ralloc.h
index db8fc7d..55e62b1 100644
--- a/src/compiler/codegen/Ralloc.h
+++ b/src/compiler/codegen/Ralloc.h
@@ -202,10 +202,7 @@
 extern void oatRecordFpPromotion(CompilationUnit* cUnit, int reg, int sReg);
 
 
-/*
- * Architecture-dependent register allocation routines implemented in
- * ${TARGET_ARCH}/${TARGET_ARCH_VARIANT}/Ralloc.c
- */
+/* Architecture-dependent register allocation routines. */
 extern int oatAllocTypedTempPair(CompilationUnit* cUnit,
                                  bool fpHint, int regClass);