configs: Build test experimental x86_64 code

Add additional build config to test qemu-i440fx x86_64 code.

Change-Id: I63f7a6e1602728e4d5ff67f9bd702efebe315c16
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/31472
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

configs/config.emulation_qemu_x86_i440fx_x86_64

@@ -0,0 +1 @@
+CONFIG_CPU_QEMU_X86_64=y