macros: add st() macro to the masm macro package

A very easy way to improve portability.  Guarded with %ifndef just in
case...

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>

doc/changes.src

@@ -67,6 +67,8 @@ It is the production version of NASM since 2025.
    implemented conditional directive, and tries to match it with a
    corresponding \c{%endif}.  See \k{if_caveat}.
 
+\b The \c{masm} macro package now defines a macro for x87 register syntax.
+
 
 \H{cl-2.xx} NASM 2 Series
 

doc/macropkg.src

@@ -178,6 +178,9 @@ after \c{PROC} other than \c{FAR} is ignored.
 \b In 64-bit mode relative addressing is the default (\c{DEFAULT REL},
 see \k{default-rel}).
 
+\b A macro is defined to allow the syntax \c{st(0)} instead of
+\c{st0}, and so on.
+
 In addition, NASM now natively supports, regardless of whether this
 package is used or not:
 

macros/masm.mac

@@ -1,6 +1,6 @@
 ;; --------------------------------------------------------------------------
 ;;
-;;   Copyright 2019 The NASM Authors - All Rights Reserved
+;;   Copyright 2019-2025 The NASM Authors - All Rights Reserved
 ;;   See the file AUTHORS included with the NASM distribution for
 ;;   the specific copyright holders.
 ;;
@@ -69,6 +69,10 @@ USE: masm
   %undef ret
 %endmacro
 
+%ifndef st
+  %idefine st(x) st %+ x
+%endif
+
 %idefine ptr __?masm_ptr?__
 %idefine flat __?masm_flat?__	; is %idefine really correct here?
 %idefine offset