src: Consolidate x86_64 support Kconfig

Introduce `USE_EXP_X86_64_SUPPORT` in `src/arch/x86/Kconfig` and guard
it with `HAVE_EXP_X86_64_SUPPORT`. Replace the per-CPU implementations
of the same functionality with the newly-added Kconfig options. Update
documentation and the config file for QEMU accordingly.

Change-Id: I550216fd2a8323342d6b605306b0b95ffd5dcd1c
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55760
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

src/cpu/intel/model_206ax/Kconfig

@@ -3,16 +3,11 @@ config CPU_INTEL_MODEL_206AX
 
 if CPU_INTEL_MODEL_206AX
 
-config ARCH_EXP_X86_64
-	bool "Experimental 64bit support"
-	depends on USE_NATIVE_RAMINIT
-	default n
-
 config CPU_SPECIFIC_OPTIONS
 	def_bool y
-	select ARCH_ALL_STAGES_X86_64 if ARCH_EXP_X86_64
 	select ARCH_X86
 	select BOOT_DEVICE_SPI_FLASH_NO_EARLY_WRITES
+	select HAVE_EXP_X86_64_SUPPORT if USE_NATIVE_RAMINIT
 	select MMX
 	select SSE2
 	select UDELAY_TSC