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/amd/agesa/Kconfig b/src/cpu/amd/agesa/Kconfig
index 51b61a1..f71b8ed 100644
--- a/src/cpu/amd/agesa/Kconfig
+++ b/src/cpu/amd/agesa/Kconfig
@@ -7,6 +7,7 @@
 	default y if CPU_AMD_AGESA_FAMILY16_KB
 	default n
 	select ARCH_ALL_STAGES_X86_32
+	select ARCH_X86
 	select DRIVERS_AMD_PI
 	select TSC_SYNC_LFENCE
 	select UDELAY_LAPIC