src: Introduce ARCH_ALL_STAGES_X86

Introduce the `ARCH_ALL_STAGES_X86` Kconfig symbol to automatically
select the per-stage arch options. Subsequent commits will leverage
this to allow choosing between 32-bit and 64-bit coreboot where all
stages are x86. AMD Picasso and AMD Cezanne are the only exceptions
to this rule: they disable `ARCH_ALL_STAGES_X86` and explicitly set
the per-stage arch options accordingly.

Change-Id: Ia2ddbae8c0dfb5301352d725032f6ebd370428c9
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55759
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

src/cpu/intel/model_206ax/Kconfig

@@ -10,7 +10,6 @@ config ARCH_EXP_X86_64
 
 config CPU_SPECIFIC_OPTIONS
 	def_bool y
-	select ARCH_ALL_STAGES_X86_32 if !ARCH_EXP_X86_64
 	select ARCH_ALL_STAGES_X86_64 if ARCH_EXP_X86_64
 	select ARCH_X86
 	select BOOT_DEVICE_SPI_FLASH_NO_EARLY_WRITES