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/soc/example/min86/Kconfig

@@ -15,6 +15,7 @@ if SOC_EXAMPLE_MIN86
 config SOC_SPECIFIC_OPTIONS
 	def_bool y
 	select ARCH_ALL_STAGES_X86_32
+	select ARCH_X86
 	select NO_MONOTONIC_TIMER
 	select NO_MMCONF_SUPPORT
 	select UNKNOWN_TSC_RATE