arch/x86: Make X86 stages select ARCH_X86

Also, don't define the default as this results in spurious lines in the
.config.

TEST: Build all boards with where config.h differed with
BUILD_TIMELESS=1 and remained the same

Change-Id: Ic77b696f493d7648f317f0ba0a27fdee5212961e
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/31316
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/sapphire/pureplatinumh61/Kconfig

@@ -2,7 +2,6 @@ if BOARD_SAPPHIRE_PUREPLATINUMH61
 
 config BOARD_SPECIFIC_OPTIONS
 	def_bool y
-	select ARCH_X86
 	select NORTHBRIDGE_INTEL_SANDYBRIDGE
 	select USE_NATIVE_RAMINIT
 	select SOUTHBRIDGE_INTEL_BD82X6X