xen/arm: Workaround clang/armclang support for register allocation

xen/arm: Workaround clang/armclang support for register allocation

Clang 8.0 (see [1]) and by extent some of the version of armclang does
not support register allocation using the syntax rN.

Thankfully, both GCC [2] and clang are able to support the xN syntax for
Arm64. Introduce a new macro ASM_REG() and use in common code for
register allocation.

[1] https://reviews.llvm.org/rL328829
[2] https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html

Cc: Andrii Anisov <andrii_anisov@epam.com>
Signed-off-by: Julien Grall <julien@xen.org>
Tested-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>