aarch64: Add codegen support for AdvSIMD faminmax

The AArch64 FEAT_FAMINMAX extension is optional from Armv9.2-a and
mandatory from Armv9.5-a. It introduces instructions for computing the
floating point absolute maximum and minimum of the two vectors
element-wise.

This patch adds code generation support for famax and famin in terms of
existing RTL operators.

famax/famin is equivalent to first taking abs of the operands and then
taking smax/smin on the results of abs.

	famax/famin (a, b) = smax/smin (abs (a), abs (b))

This fusion of operators is only possible when -march=armv9-a+faminmax
flags are passed. We also need to pass -ffast-math flag; if we don't,
then a statement like

	c[i] = __builtin_fmaxf16 (a[i], b[i]);

is RTL expanded to UNSPEC_FMAXNM instead of smax (likewise for smin).

This code generation is only available on -O2 or -O3 as that is when
auto-vectorization is enabled.

gcc/ChangeLog:

	* config/aarch64/aarch64-simd.md
	(*aarch64_faminmax_fused): Instruction pattern for faminmax
	codegen.
	* config/aarch64/iterators.md: Attribute for faminmax codegen.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/simd/faminmax-codegen-no-flag.c: New test.
	* gcc.target/aarch64/simd/faminmax-codegen.c: New test.
	* gcc.target/aarch64/simd/faminmax-no-codegen.c: New test.