Change the FMV rules so target_clones defaults must be explicitly stated

This is needed to disambiguate cases such as:

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

int foo() { return 1; }
```

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

Where with the previous specification two dispatchers would be created
and two defaults with different contents.

This is fixed by requiring default versions in target_clones to be
explicitly stated. This makes it bery simple to determine where the
dispatcher should be created accross translation units.

Author: AlfieRichardsArm

main/acle.md

@@ -424,6 +424,7 @@ Armv8.4-A [[ARMARMv84]](#ARMARMv84). Support is added for the Dot Product intrin
 * Removed Function Multi Versioning feature dgh.
 * Document Function Multi Versioning feature dependencies.
 * Simplified Function Multi Versioning version selection rules.
+* Changed the Function Multi Versioning default version rules to be more explicit.
 * Fixed range of operand `o0` (too small) in AArch64 system register designations.
 * Fixed SVE2.1 quadword gather load/scatter store intrinsics.
 * Removed unnecessary Zd argument from `svcvtnb_mf8[_f32_x2]_fpm`.
@@ -439,7 +440,6 @@ Armv8.4-A [[ARMARMv84]](#ARMARMv84). Support is added for the Dot Product intrin
 * Changed the status of the SME2p1 ACLE from Alpha to Beta.
 * Changed the status of the SVE2p1 ACLE from Alpha to Beta.
 
-
 ### References
 
 This document refers to the following documents.
@@ -2704,17 +2704,19 @@ 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`.
@@ -2741,11 +2743,6 @@ 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