aarch64: Add support for fmv priority syntax.

aarch64: Add support for fmv priority syntax.

Adds support for the AArch64 fmv priority syntax.

This allows users to override the default function ordering.

For example:

```c
int bar [[gnu::target_version("default")]] (int){
  return 1;
}

int bar [[gnu::target_version("dotprod;priority=2")]] (int) {
  return 2;
}

int bar [[gnu::target_version("sve;priority=1")]] (int) {
  return 3;
}
```

gcc/ChangeLog:

	* config/aarch64/aarch64.cc (aarch64_parse_fmv_features): Add parsing
	for priority arguments.
	(aarch64_process_target_version_attr): Update call to
	aarch64_parse_fmv_features.
	(get_feature_mask_for_version): Update call to
	aarch64_parse_fmv_features.
	(aarch64_compare_version_priority): Add logic to order by priority if present.
	(aarch64_functions_b_resolvable_from_a): Update call to
	aarch64_parse_fmv_features.
	(aarch64_mangle_decl_assembler_name): Update call to
	aarch64_parse_fmv_features.
	(dispatch_function_versions): Add logic to sort by priority.
	(aarch64_same_function_versions): Add diagnostic if invalid use of
	priority syntax.
	(aarch64_merge_decl_attributes): Add logic to make suer priority
	arguments are preserved.
	(aarch64_check_target_clone_version): Update call to
	aarch64_parse_fmv_features.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/fmv_priority3.c: New test.
	* gcc.target/aarch64/fmv_priority_error1.c: New test.
	* gcc.target/aarch64/fmv_priority_error2.c: New test.