configs/rdn2cfg3: add x86_64 host specific config fragment for buildroot

When building buildroot on a x86_64 host, BR2_TOOLCHAIN_EXTERNAL should
be set to y to indicate that an external toolchain should be used to
build for aarch64 target. Add a new x86_64 host specific config fragment
for buildroot and move out the x86_64 host specific config into this
file from the defconfig file.

Signed-off-by: Tony Nadackal <tony.nadackal@arm.com>
Change-Id: I78ae9d6724d5cc206aed926352a50b51e8a8bf4d