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
diff --git a/build/tasks/kernel.mk b/build/tasks/kernel.mk
index 5025218..ec26ded 100644
--- a/build/tasks/kernel.mk
+++ b/build/tasks/kernel.mk
@@ -269,6 +269,12 @@
 else
     KERNEL_CROSS_COMPILE := CROSS_COMPILE="$(ccache) $(KERNEL_TOOLCHAIN_PATH)"
 endif
+
+# Needed for CONFIG_COMPAT_VDSO, safe to set for all arm64 builds
+ifeq ($(KERNEL_ARCH),arm64)
+   KERNEL_CROSS_COMPILE += CROSS_COMPILE_ARM32="arm-linux-androideabi-"
+endif
+
 ccache =
 
 ifeq ($(HOST_OS),darwin)