kernel: Set CROSS_COMPILE_ARM32 if using aarch64

Google added a 32-bit vDSO to the Wahoo and Marlin kernels on Android P,
requiring a 32-bit toolchain to compile/link, and the build fails when
it is not provided.

Change-Id: I700e66a417ed431c31d82fc950f5e5acd07ab281
1 file changed