arm64: vdso32: Move definition of COMPATCC into vdso32/Makefile

There's no need to export COMPATCC, so just define it locally in the
vdso32/Makefile, which is the only place where it is used.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>