arm64: Enable compat vDSO support

This patch enables compat vDSO support in the arm64 defconfig.

Note that the default compiler is set to arm-linux-gnueabihf-.
For this to take effect and for the compat vDSO to compile correctly, it
is required that this compiler is in the PATH.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>