arm64: vdso: Don't use gcc plugins for building vgettimeofday.c

Don't use gcc plugins for building arch/arm64/kernel/vdso/vgettimeofday.c
to avoid unneeded instrumentation.

Signed-off-by: Alexander Popov <alex.popov@linux.com>
Link: https://lore.kernel.org/r/20200624123330.83226-4-alex.popov@linux.com


Signed-off-by: Will Deacon <will@kernel.org>