arm64: compat: vdso: Expose BUILD_VDSO32

clock_gettime32 and clock_getres_time32 should be compiled only with the
32 bit vdso library.

Expose BUILD_VDSO32 when arm64 compat is compiled, to provide an
indication to the generic library to include these symbols.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20190830135902.20861-2-vincenzo.frascino@arm.com