Change the FMV rules so target_clones default versions must be explicitly stated (#377)

This patch requires default versions in target_clones to be explicitly stated.

This makes it simple to determine where the dispatcher should be created
across translation units, and resolves cases (such as below) where we
can end up making multiple dispatchers.

// Translation unit 1
__attribute__ ((target_clone("dotprod, sve"))) int foo();

int foo() { return 1; }

// Translation unit 2
__attribute__ ((target_clone("dotprod, sve"))) int foo() { return 2; }

With the previous specification, my understanding is, the second translation
unit would implicitly add a default. Then there would be two implementations
of defaults, and so two dispatchers would be created.

Author: AlfieRichardsArm

main/acle.md

@@ -445,6 +445,7 @@ Armv8.4-A [[ARMARMv84]](#ARMARMv84). Support is added for the Dot Product intrin
 * Added `svdot[_n_f16_mf8]_fpm` and `svdot[_n_f32_mf8]_fpm`.
 * Added Guarded Control Stack (GCS) at
   [**Beta**](#current-status-and-anticipated-changes) quality level.
+* Changed the Function Multi Versioning default version rules to be more explicit.
 
 ### References
 
@@ -2721,17 +2722,19 @@ The following attributes trigger the multi version code generation:
   type of the default version.
 * All the function versions must be declared at the translation
   unit in which the definition of the default version resides.
+* One `default` version of the function is required to be provided
+  in one of the translation units.
+  * Implicitly, as a definition without any attribute,
+  * as a function annotated with `target_version("default")`,
+  * or, as a function annotated with `target_clones(...)` where one
+    of the versions is `default`.
 
 The attribute `__attribute__((target_version("name")))` expresses the
 following:
 
 * When applied to a function it becomes one of the versions.
 * Multiple function versions may exist in the same or in different
   translation units.
-* One `default` version of the function is required to be provided
-  in one of the translation units.
-  * Implicitly, without this attribute,
-  * or explicitly providing the `default` in the attribute.
 
 For example, the below is valid and 2 is used as the default
 value for `c` when calling the multiversioned function `f`.
@@ -2758,11 +2761,6 @@ following:
 
 * when applied to a function the compiler emits multiple versions
   based on the arguments.
-  * One of them is implicitly the `default`.
-  * If the `default` matches with another explicitly provided
-    version in the same translation unit, then the compiler can
-    emit only one function instead of the two. The explicitly
-    provided version shall be preferred.
 * If a name is not recognized the compiler should ignore it[^fmv-note-ignore].
 
 [^fmv-note-ignore]: The intention is to support the usecase of newer code if