[clang][doc] Document and prefer __asm as mangled name attribute (llvm#167226)

It's supported together with the other spellings and results in the same attribute.
Document it and prefer it in the documentation as the `asm()` spelling is C++ and GNU-only.

See: llvm#167221 (comment)

Author: tambry

clang/include/clang/Basic/Attr.td

@@ -1069,7 +1069,7 @@ def AVRSignal : InheritableAttr, TargetSpecificAttr<TargetAVR> {
 }
 
 def AsmLabel : InheritableAttr {
-  let Spellings = [CustomKeyword<"asm">, CustomKeyword<"__asm__">];
+  let Spellings = [CustomKeyword<"asm">, CustomKeyword<"__asm">, CustomKeyword<"__asm__">];
   let Args = [
       // Label specifies the mangled name for the decl.
       StringArgument<"Label">, ];

clang/include/clang/Basic/AttrDocs.td

@@ -4295,17 +4295,17 @@ used by other languages. (This prefix is also added to the standard Itanium
 C++ ABI prefix on "mangled" symbol names, so that e.g. on such targets the true
 symbol name for a C++ variable declared as ``int cppvar;`` would be
 ``__Z6cppvar``; note the two underscores.)  This prefix is *not* added to the
-symbol names specified by the ``asm`` attribute; programmers wishing to match a
-C symbol name must compensate for this.
+symbol names specified by the ``__asm`` attribute; programmers wishing to match
+a C symbol name must compensate for this.
 
 For example, consider the following C code:
 
 .. code-block:: c
 
-  int var1 asm("altvar") = 1;  // "altvar" in symbol table.
+  int var1 __asm("altvar") = 1;  // "altvar" in symbol table.
   int var2 = 1; // "_var2" in symbol table.
 
-  void func1(void) asm("altfunc");
+  void func1(void) __asm("altfunc");
   void func1(void) {} // "altfunc" in symbol table.
   void func2(void) {} // "_func2" in symbol table.