Relax the naming rule of extension name

Current spec only allow alphabetical name, which means we can't use number in
the name, but we already defined serveral extension name with number,
like `zve32x`, `zvl128b`.

src/naming.tex

@@ -77,10 +77,10 @@ \section{Underscores}
 \section{Additional Standard Extension Names}
 
 Standard extensions can also be named using a single ``Z'' followed by an
-alphabetical name and an optional version number.  For example,
-``Zifencei'' names the instruction-fetch fence extension described in
-Chapter~\ref{chap:zifencei}; ``Zifencei2'' and ``Zifencei2p0'' name version
-2.0 of same.
+alphanumeric name, the name must end with an alphabetical, and an optional
+version number.  For example, ``Zifencei'' names the instruction-fetch fence
+extension described in Chapter~\ref{chap:zifencei}; ``Zifencei2'' and
+``Zifencei2p0'' name version 2.0 of same.
 
 The first letter following the ``Z'' conventionally indicates the most closely
 related alphabetical extension category, IMAFDQLCBKJTPV.  For the ``Zam''
@@ -96,8 +96,9 @@ \section{Additional Standard Extension Names}
 \section{Supervisor-level Instruction-Set Extensions}
 
 Standard supervisor-level instruction-set extensions are defined in Volume II,
-but are named using ``S'' as a prefix, followed by an alphabetical name and an
-optional version number.  Supervisor-level extensions must be separated from
+but are named using ``S'' as a prefix, followed by an alphanumeric name, the
+name must end with an alphabetical, and an optional version number.
+Supervisor-level extensions must be separated from
 other multi-letter extensions by an underscore.
 
 Standard supervisor-level extensions should be listed after standard
@@ -126,7 +127,8 @@ \section{Machine-level Instruction-Set Extensions}
 \section{Non-Standard Extension Names}
 
 Non-standard extensions are named using a single ``X'' followed by an
-alphabetical name and an optional version number.
+alphanumeric name, the name must end with an alphabetical, and an optional
+version number.
 For example, ``Xhwacha'' names the Hwacha vector-fetch ISA extension;
 ``Xhwacha2'' and ``Xhwacha2p0'' name version 2.0 of same.