Minor modification to FMV rules for scope and signatures

[AlfieRichardsArm]

Hi all,

While attempting to implement FMV in the GCC front-end some questions were raised that I think are worth clarifying here.

This PR changes the rules to use the default function to determine the signature and scope of the versioned function set.

This clears up some cases such as:

int fn (int c = 1);
int __attribute__((target_version("sve"))) fn (int c = 2);

int bar() { return fn(); }

Where there are conflicting signatures and which default should be used is not clear at the moment.

int fn (int c[]);
int __attribute__((target_version("default"))) fn (int c[1]) {
}
int __attribute__((target_version("sve"))) fn (int c[2]) {
}

Where if this should be considered a conflicting signature is not clear.

int __attribute__((target_version("default"))) fn (int x) {
    return 1;
}

void bar () {
    int __attribute__((target_version("sve2"))) fn (int);
    fn(1);
}

Where the scope of multi-versioned functions differs.

And

// TU 1
#import TU2

int fn (int c = 1);

int bar() { return fn(); }

// TU 2

int __attribute__((target_version("sve"))) fn (int c = 2);
int __attribute__((target_version("sve2"))) fn (int c = 2);

int bar() { return fn(); }

Where it is possible calls in different TU's could use different default argument values.

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[AlfieRichardsArm]

I have updated this PR with (hopefully) clearer wording more inline with @labrinea 's suggestions.

[AlfieRichardsArm]

I have changed the rule for whats allowed signature wise after a suggestion by @rsandifo-arm which makes it more precise in terms of function types and added some examples.

Specifically, the rule change specifies compatibility in terms of if the types are compatible, and the example shows this in terms of function pointers.
The argument there being that behind the scenes FMV is the load time initialization of a function pointer, so it makes sense to consider compatibility in terms of function pointer compatibility.

[vhscampos]

If no objections are raised, let's merge this on the 23 December.