arm: Fix MVE intrinsics support with LTO (PR target/110268)

After the recent MVE intrinsics re-implementation, LTO stopped working
because the intrinsics would no longer be defined.

The main part of the patch is simple and similar to what we do for
AArch64:
- call handle_arm_mve_h() from arm_init_mve_builtins to declare the
  intrinsics when the compiler is in LTO mode
- actually implement arm_builtin_decl for MVE.

It was just a bit tricky to handle __ARM_MVE_PRESERVE_USER_NAMESPACE:
its value in the user code cannot be guessed at LTO time, so we always
have to assume that it was not defined.  The led to a few fixes in the
way we register MVE builtins as placeholders or not.  Without this
patch, we would just omit some versions of the inttrinsics when
__ARM_MVE_PRESERVE_USER_NAMESPACE is true. In fact, like for the C/C++
placeholders, we need to always keep entries for all of them to ensure
that we have a consistent numbering scheme.

2023-06-26  Christophe Lyon   <christophe.lyon@linaro.org>

	PR target/110268
	gcc/
	* config/arm/arm-builtins.cc (arm_init_mve_builtins): Handle LTO.
	(arm_builtin_decl): Hahndle MVE builtins.
	* config/arm/arm-mve-builtins.cc (builtin_decl): New function.
	(add_unique_function): Fix handling of
	__ARM_MVE_PRESERVE_USER_NAMESPACE.
	(add_overloaded_function): Likewise.
	* config/arm/arm-protos.h (builtin_decl): New declaration.

	gcc/testsuite/
	* gcc.target/arm/pr110268-1.c: New test.
	* gcc.target/arm/pr110268-2.c: New test.

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