UPSTREAM: 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>
(cherry picked from commit bcaf9b57e4884e86717c1f4cee8157fd68189aa7)
Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 154668398
Change-Id: Id9eb2cfe1e13a6858f42c8ff025bb5fc9bf096a7