arm64: vdso: Build vDSO with -ffixed-x18

The vDSO needs to be built 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 using the generic C implementation of the vDSO.

[ tglx: Massaged changelog ]

Signed-off-by: Peter Collingbourne <[email protected]>
Signed-off-by: Vincenzo Frascino <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Tested-by: Shijith Thotton <[email protected]>
Tested-by: Andre Przywara <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: Catalin Marinas <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Russell King <[email protected]>
Cc: Ralf Baechle <[email protected]>
Cc: Paul Burton <[email protected]>
Cc: Daniel Lezcano <[email protected]>
Cc: Mark Salyzyn <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Dmitry Safonov <[email protected]>
Cc: Rasmus Villemoes <[email protected]>
Cc: Huw Davies <[email protected]>
Cc: Mark Salyzyn <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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