xen/arm: domain: Fix MISRA C 2012 Rule 8.7 violation

xen/arm: domain: Fix MISRA C 2012 Rule 8.7 violation

The function idle_loop() is referenced only in domain.c.
Change its linkage from external to internal by adding the storage-class
specifier static to its definitions.

Add the function as a 'fake' input operand to the inline assembly statement,
to make the compiler aware that the function is used.
Fake means that the function is not actually used as an operand by the asm code.
That is because there is not a suitable gcc arm32 asm constraint for labels.

Declare return_to_new_vcpu32() and return_to_new_vcpu64() that are also
referenced by this inline asm statement.

Also, this patch resolves indirectly a MISRA C 2012 Rule 8.4 violation warning.

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
[add noreturn]
Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>