versions: add D_REG64, remove D_X32, document X32

Author: WalterBright

spec/version.dd

@@ -264,6 +264,7 @@ $(H3 $(LEGACY_LNAME2 PredefinedVersions, predefined-versions, Predefined Version
         $(TROW $(ARGS $(D CppRuntime_Sun)) , $(ARGS Sun Cpp runtime))
         $(TROW $(ARGS $(D X86)) , $(ARGS Intel and AMD 32-bit processors))
         $(TROW $(ARGS $(D X86_64)) , $(ARGS Intel and AMD 64-bit processors))
+        $(TROW $(ARGS $(D X32)) , $(ARGS Set when D_LP64 is not set and X86_64 is not set. Do not use.))
         $(TROW $(ARGS $(D ARM)) , $(ARGS The ARM architecture (32-bit) (AArch32 et al)))
         $(TROW $(ARGS $(D ARM_Thumb)) , $(ARGS ARM in any Thumb mode))
         $(TROW $(ARGS $(D ARM_SoftFloat)) , $(ARGS The ARM $(D soft) floating point ABI))
@@ -327,7 +328,7 @@ $(H3 $(LEGACY_LNAME2 PredefinedVersions, predefined-versions, Predefined Version
         $(TROW $(ARGS $(D D_InlineAsm_X86_64)) , $(ARGS $(DDLINK spec/iasm, Inline Assembler, Inline assembler) for X86-64 is implemented))
         $(TROW $(ARGS $(D D_LP64)) , $(ARGS $(B Pointers) are 64 bits
                 (command line switch $(DDSUBLINK dmd, switch-m64, $(TT -m64))). (Do not confuse this with C's LP64 model)))
-        $(TROW $(ARGS $(D D_X32)) , $(ARGS Pointers are 32 bits, but words are still 64 bits (x32 ABI) (This can be defined in parallel to $(D X86_64))))
+        $(TROW $(ARGS $(D D_REG64)) , $(ARGS Set when the CPU has 64 bit registers))
         $(TROW $(ARGS $(D D_HardFloat)) , $(ARGS The target hardware has a floating-point unit))
         $(TROW $(ARGS $(D D_SoftFloat)) , $(ARGS The target hardware does not have a floating-point unit))
         $(TROW $(ARGS $(D D_PIC)) , $(ARGS Position Independent Code