BACKPORT: arm64: Add support for relocating the kernel with RELR relocations

RELR is a relocation packing format for relative relocations.
The format is described in a generic-abi proposal:
https://groups.google.com/d/topic/generic-abi/bX460iggiKg/discussion

The LLD linker can be instructed to pack relocations in the RELR
format by passing the flag --pack-dyn-relocs=relr.

This patch adds a new config option, CONFIG_RELR. Enabling this option
instructs the linker to pack vmlinux's relative relocations in the RELR
format, and causes the kernel to apply the relocations at startup along
with the RELA relocations. RELA relocations still need to be applied
because the linker will emit RELA relative relocations if they are
unrepresentable in the RELR format (i.e. address not a multiple of 2).

Enabling CONFIG_RELR reduces the size of a defconfig kernel image
with CONFIG_RANDOMIZE_BASE by 3.5MB/16% uncompressed, or 550KB/5%
compressed (lz4).

Signed-off-by: Peter Collingbourne <pcc@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Will Deacon <will@kernel.org>
(cherry picked from commit 5cf896fb6be3effd9aea455b22213e27be8bdb1d)
Bug: 137200966
Test: booted defconfig + CONFIG_RELR kernel on qemu
Change-Id: I4c55bf5b10bc6c934543c651eca9fc8e260ffc6d
[pcc: 4.14 lacks support for running shell commands in Kconfig, so
CONFIG_TOOLS_SUPPORT_RELR is made user-configurable and users must
self-declare tool support]
Signed-off-by: Peter Collingbourne <pcc@google.com>
6 files changed