arm: [MVE intrinsics] Avoid warnings when floating-point is not supported [PR 117814]

arm: [MVE intrinsics] Avoid warnings when floating-point is not supported [PR 117814]

If the target does not support floating-point, we register FP vector
types as 'void' (see register_vector_type).

This leads to warnings about 'pure attribute on function returning
void' when we declare the various load intrinsics because their
call_properties say CP_READ_MEMORY (thus giving them the 'pure'
attribute), but their return type is void.  This happens for instance
in gcc.target/arm/pr112337.c, depending on how GCC is built (I didn't
notice the warnings because arm_mve.h is considered as a system
include in my environment, and the warning is not emitted, but CI
reported it).

To avoid such warnings, declare floating-point scalar and vector types
even if the target does not have an FPU.

Note that since an FPU can be activated via #pragma GCC target
("arch=armv8.1-m.main+mve.fp" for instance), it means that such types
cannot appear and disappear withing a single TU, they have to be
available in all contexts.  This implies a noteworthy change for
__fp16: it not longer depends on using -mfp16-format=ieee or
alternative.  Also note that if the target ISA has the fp16 bit set,
we already silently activate -mfp16-format=ieee (with an error if
-mfp16-format=alternative was supplied).  The patch now enforces
-mfp16-format=none if the option was used.

In arm-mve-builtins.cc (register_builtin_types, register_vector_type,
register_builtin_tuple_types), this means simply removing the early
exits.  However, for this to work, we need to update
arm_vector_mode_supported_p, so that vector floating-point types are
always defined, and __fp16 must always be registered by
arm_init_fp16_builtins (as it is the base type for vectors of
float16_t.  Another side effect is that the declaration of float16_t
and float32_t typedefs is now unconditional.

The new tests verify that:
- we emit an error if the code tries to use floating-point intrinsics
  and the target does not have the floating-point extension
- we emit the expected code when activating the floating-point
  expected via a pragma
- we emit the expected code when the target supports floating-point
  (no pragma needed)
- we apply -mfp16-format=none where we used to default to ieee

An update is needed in g++.target/arm/mve/general-c++/nomve_fp_1.c,
because the error message now correctly uses float16x8_t instead of
void as return type.

The patch removes gcc.target/arm/fp16-compile-none-1.c which tests
that using __fp16 produces an error with -mfp16-format=none, since it
is no longer the case.

gcc/ChangeLog:

	PR target/117814
	* config/arm/arm-builtins.cc (arm_init_fp16_builtins): Always
	register __fp16 type.
	* config/arm/arm-mve-builtins.cc (register_builtin_tuple_types):
	Remove special handling when TARGET_HAVE_MVE_FLOAT is false.
	(register_vector_type): Likewise.
	(register_builtin_tuple_types): Likewise.
	* config/arm/arm-opts.h (arm_fp16_format_type): Add
	ARM_FP16_FORMAT_DEFAULT.
	* config/arm/arm.cc (arm_vector_mode_supported_p): Accept
	floating-point vector modes even if TARGET_HAVE_MVE_FLOAT is
	false.
	(arm_option_reconfigure_globals): Apply ARM_FP16_FORMAT_NONE if
	requested.
	* config/arm/arm.opt (mfp16-format): Default to
	ARM_FP16_FORMAT_DEFAULT.
	* config/arm/arm_mve_types.h (float16_t, float32_t): Define
	unconditionally.
	* doc/sourcebuild.texi (ARM-specific attributes): Document
	arm_v8_1m_mve_nofp_ok.

gcc/testsuite/ChangeLog:

	PR target/117814
	* gcc.target/arm/mve/intrinsics/pr117814-f16.c: New test.
	* gcc.target/arm/mve/intrinsics/pr117814-2-f16.c: New test.
	* gcc.target/arm/mve/intrinsics/pr117814-3-f16.c: New test.
	* gcc.target/arm/mve/intrinsics/pr117814-4-f16.c: New test.
	* gcc.target/arm/mve/intrinsics/pr117814-f32.c: New test.
	* gcc.target/arm/mve/intrinsics/pr117814-2-f32.c: New test.
	* gcc.target/arm/mve/intrinsics/pr117814-3-f32.c: New test.
	* gcc.target/arm/fp16-compile-none-1.c: Delete.
	* g++.target/arm/mve/general-c++/nomve_fp_1.c: Fix expected error
	message.
	* lib/target-supports.exp
	(check_effective_target_arm_v8_1m_mve_nofp_ok_nocache): New.
	(check_effective_target_arm_v8_1m_mve_nofp_ok): New.
	(add_options_for_arm_v8_1m_mve_nofp): New.