* Generate a nop instruction after ais annotations.

In order to prevent the merging of ais annotations with following
Labels a nop instruction is inserted, but only if the annotation
is followed immediately by a label.

The insertion of nop instructions is performed during the
expansion of builtin and pseudo assembler instructions and is
processor independent, by inserting a __builtin_nop built-in.

* Add Pnop instruction to ARM, RISC-V, and x86

ARM as well as RISC-V don't have nop instructions that can
be easily encoded by for example add with zero instructions.
For x86 we used to use `mov X0, X0` for nop but this may
not be as efficient as the true nop instruction.

* Implement __builtin_nop on all supported target architectures.

This builtin is not yet made available on the C side for all architectures.

Bug 24067