We already have support for a wide range of x86-64 CPUs, but we don't
have any option to build for the most generic possible x86-64 CPU, as
made available by the -march=x86-64 GCC option.
This commit makes this option available in Buildroot.
It only really makes sense to use as a 64-bit CPU, and not as a 32-bit
one, so we guard it behind BR2_x86_64.
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[yann.morin.1998@free.fr: it's an x86-64-only CPU]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>