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>
diff --git a/src/cpu/qemu-x86/Kconfig b/src/cpu/qemu-x86/Kconfig
index 674e05c..440d8cb 100644
--- a/src/cpu/qemu-x86/Kconfig
+++ b/src/cpu/qemu-x86/Kconfig
@@ -2,6 +2,7 @@
 
 config CPU_QEMU_X86
 	bool
+	select ARCH_X86
 	select UDELAY_TSC
 	select TSC_MONOTONIC_TIMER
 	select UNKNOWN_TSC_RATE