FROMLIST: arm64: Build vDSO with -ffixed-x18

https://patchwork.kernel.org/patch/10671685/

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/



Signed-off-by: Peter Collingbourne <pcc@google.com>
Cc: Mark Salyzyn <salyzyn@google.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Bug: 112907825
Change-Id: I175688a57af877a7aaef6b6d95be4d36ad52f8fc
Signed-off-by: Albert I <kras@raphielgang.org>
Signed-off-by: Jprimero15 <jprimero155@gmail.com>