FROMLIST: arm64: build vDSO with -ffixed-x18

The vDSO needs to be build with x18 reserved in order to accommodate
userspace platform ABIs built on top of Linux that use the register
to carry inter-procedural state, as provided for by the AAPCS.
An example of such a platform ABI is the one that will be used by an
upcoming version of Android.

Although this change is currently a no-op due to the fact that the vDSO
is currently implemented in pure assembly on arm64, it is necessary
in order to prepare for another change [1] that will add C code to
the vDSO.

[1] https://patchwork.kernel.org/patch/10044501/

Change-Id: Icaac4b1c9127d81d754d3b8688274e9afc781760
Signed-off-by: Peter Collingbourne <pcc@google.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Mark Salyzyn <salyzyn@google.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Khusika Dhamar Gusti <khusikadhamar@gmail.com>
Signed-off-by: TheSync <repo-sync@outlook.com>
Signed-off-by: Azuki <id_azuki@protonmail.com>

arch/arm64/kernel/vdso/Makefile

@@ -15,7 +15,7 @@ obj-vdso-c := $(addprefix $(obj)/, $(obj-vdso-c))
 obj-vdso   := $(obj-vdso-c) $(obj-vdso-s)
 
 ccflags-y := -shared -fno-common -fno-builtin -fno-stack-protector
-ccflags-y += -DDISABLE_BRANCH_PROFILING
+ccflags-y += -DDISABLE_BRANCH_PROFILING -ffixed-x18
 ccflags-y += -nostdlib -Wl,-soname=linux-vdso.so.1 \
 		$(call cc-ldoption, -Wl$(comma)--hash-style=sysv)
 ccflags-y += $(DISABLE_LTO)