It is currently impossible to use fp16 on any architecture higher than Armv8.3-a
due to a bug in options canonization.  This bug results in the fp16 flag not
being emitted in the assembly when it should have been.

This is caused by a complicated architectural requirement at Armv8.4-a.  On
Armv8.2-a and Armv8.3-a fp16fml is an optional extension and turning it on turns
on both fp and fp16.  However starting with Armv8.4-a fp16fml is mandatory if
fp16 is available, otherwise it's optional.

In short this means that to enable fp16fml the smallest option that needs to
passed to the assembler is Armv8.4-a+fp16.

The fix in this patch takes into account that an option may be on by default in
an architecture, but that not all the bits required to use it are on by default
in an architecture.  In such cases the difference between the two are still
emitted to the assembler.

gcc/ChangeLog:

	PR target/94396
	* common/config/aarch64/aarch64-common.c
	(aarch64_get_extension_string_for_isa_flags): Handle default flags.

gcc/testsuite/ChangeLog:

	PR target/94396
	* gcc.target/aarch64/options_set_11.c: New test.
	* gcc.target/aarch64/options_set_12.c: New test.
	* gcc.target/aarch64/options_set_13.c: New test.
	* gcc.target/aarch64/options_set_14.c: New test.
	* gcc.target/aarch64/options_set_15.c: New test.
	* gcc.target/aarch64/options_set_16.c: New test.
	* gcc.target/aarch64/options_set_17.c: New test.
	* gcc.target/aarch64/options_set_18.c: New test.
	* gcc.target/aarch64/options_set_19.c: New test.
	* gcc.target/aarch64/options_set_20.c: New test.
	* gcc.target/aarch64/options_set_21.c: New test.
	* gcc.target/aarch64/options_set_22.c: New test.
	* gcc.target/aarch64/options_set_23.c: New test.
	* gcc.target/aarch64/options_set_24.c: New test.
	* gcc.target/aarch64/options_set_25.c: New test.
	* gcc.target/aarch64/options_set_26.c: New test.