src: Move select ARCH_X86 to platforms

To generalise the choice of 32-bit or 64-bit coreboot on x86 hardware,
have platforms select `ARCH_X86` directly instead of through per-stage
Kconfig options, effectively reversing the dependency order.

Change-Id: If15436817ba664398055e9efc6c7c656de3bf3e4
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55758
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

src/cpu/intel/model_106cx/Kconfig

@@ -1,6 +1,7 @@
 config CPU_INTEL_MODEL_106CX
 	bool
 	select ARCH_ALL_STAGES_X86_32
+	select ARCH_X86
 	select SSE2
 	select UDELAY_TSC
 	select TSC_MONOTONIC_TIMER