Add x86_64 variants to BR2_GCC_TARGET_ARCH

With the Sourcery CodeBench IA32/AMD64 toolchain, the proper -march=
switch must be passed. So, on x86_64, we make sure that
BR2_GCC_TARGET_ARCH gets defined to the correct value, just as we do
on x86.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>