[clang] [AArch64] Implement FP8 floating-point mode helper intrinsics (PR #100608)

[Momchil Velikov via cfe-commits]

momchil-velikov wrote:

> > * passing `__arm_fpm_init()` as the `__fpm` to these CodeGen tests cannot check whether the correct value is used to zero-out the target bits.
> 
> Good point, in the tests where we set a field to zero we should start with a non-zero in that field.

Tweaked the test a bit.

https://github.com/llvm/llvm-project/pull/100608