kbuild: clear LDFLAGS in the top Makefile

The kernel needs to be compiled as a LP64 binary for ARM64, even when
using a compiler that defaults to code-generation for the ILP32 ABI.
Consequently, we need to explicitly pass '-mabi=lp64' (supported on
gcc-4.9 and newer).

Signed-off-by: Andrew Pinski <Andrew.Pinski@caviumnetworks.com>
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Signed-off-by: Christoph Muellner <christoph.muellner@theobroma-systems.com>
Signed-off-by: Yury Norov <ynorov@caviumnetworks.com>
Reviewed-by: David Daney <ddaney@caviumnetworks.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Change-Id: Ife00e01e3e3a6d85841b88c2f1a042617bd61500