xen/arm: alternatives: Add dynamic patching feature

xen/arm: alternatives: Add dynamic patching feature

This is based on the Linux commit dea5e2a4c5bc "arm64: alternatives: Add
dynamic patching feature" written by Marc Zyngier:

    We've so far relied on a patching infrastructure that only gave us
    a single alternative, without any way to provide a range of potential
    replacement instructions. For a single feature, this is an all or
    nothing thing.

    It would be interesting to have a more flexible grained way of patching the
    kernel though, where we could dynamically tune the code that gets injected.

    In order to achive this, let's introduce a new form of dynamic patching,
    assiciating a callback to a patching site. This callback gets source and
    target locations of the patching request, as well as the number of
    instructions to be patched.

    Dynamic patching is declared with the new ALTERNATIVE_CB and alternative_cb
    directives:
                    asm volatile(ALTERNATIVE_CB("mov %0, #0\n", callback)
                                 : "r" (v));
    or

                    alternative_cb callback
                            mov x0, #0
                    alternative_cb_end

    where callback is the C function computing the alternative.

    Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
    Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
    Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
This is part of XSA-263.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>