lib/vdso: Build 32 bit specific functions in the right context

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

Exclude these symbols when BUILD_VDSO32 is not defined.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Andy Lutomirski <luto@kernel.org>
Link: https://lore.kernel.org/r/20190830135902.20861-3-vincenzo.frascino@arm.com