cpu/x86/Kconfig: Mark 64bit support as stable

With SMM holding page tables itself, we can consider SMM support stable
and safe enough for general use.

Also update the respective documentation.

Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Change-Id: Ifcf0a1a5097a2d7c064bb709ec0b09ebee13a47d
Reviewed-on: https://review.coreboot.org/c/coreboot/+/80338
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin L Roth <gaumless@gmail.com>
diff --git a/configs/config.lenovo_x201_all_debug_option_table_bt_on_wifi b/configs/config.lenovo_x201_all_debug_option_table_bt_on_wifi
index 63becd7..148c77d 100644
--- a/configs/config.lenovo_x201_all_debug_option_table_bt_on_wifi
+++ b/configs/config.lenovo_x201_all_debug_option_table_bt_on_wifi
@@ -10,4 +10,4 @@
 CONFIG_DEBUG_SPI_FLASH=y
 CONFIG_DEBUG_BOOT_STATE=y
 CONFIG_DEBUG_ADA_CODE=y
-CONFIG_USE_EXP_X86_64_SUPPORT=y
+CONFIG_USE_X86_64_SUPPORT=y