[Bug target/95471] vrndvq_f32 not supported on armv8

[evan@coeus-group.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95471

--- Comment #2 from Evan Nemerson <evan@coeus-group.com> ---
In that case shouldn't the header be adjusted to not define the vrndvq_f32
function unless it is enabled?

It is already guarded by a check for __ARM_ARCH >= 8 (see
<https://github.com/gcc-mirror/gcc/blob/95827968e5f3e472bbef0ad8b65b0a7537c9cee3/gcc/config/arm/arm_neon.h#L1757>,
but obviously that isn't sufficient.  If nothing else it would help document
the actual requirements, which would be great since the flags and macros on ARM
are a bit of a mess.