arm64: add support for building vmlinux as a relocatable PIE binary

arm64: add support for building vmlinux as a relocatable PIE binary

This implements CONFIG_RELOCATABLE, which links the final vmlinux
image with a dynamic relocation section, allowing the early boot code
to perform a relocation to a different virtual address at runtime.

This is a prerequisite for KASLR (CONFIG_RANDOMIZE_BASE).

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
(cherry picked from commit 1e48ef7fcc374051730381a2a05da77eb4eafdb0)
Signed-off-by: Alex Shi <alex.shi@linaro.org>