mb/emulation/qemu-q35,qemu-i440fx: Add x86_64 support

* Enable optional x86_64 romstage, postcar and ramstage
* Add Kconfig for x86_64 compilation
* Add documentation for x86 qemu mainboards
* Increase CAR stack as x86_64 uses more than 0x4000 bytes

Working:
* Boots to Linux
* Boots to SeaBIOS
* Drops to protected mode at end of ramstage
* Enumerates PCI devices
* Relocateable ramstage
* SMM

Change-Id: If2f02a95b2f91ab51043d4e81054354f4a6eb5d5
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/29667
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/src/cpu/qemu-x86/Kconfig b/src/cpu/qemu-x86/Kconfig
index 21ada02..af0c746 100644
--- a/src/cpu/qemu-x86/Kconfig
+++ b/src/cpu/qemu-x86/Kconfig
@@ -2,12 +2,29 @@
 
 config CPU_QEMU_X86
 	bool
-	select ARCH_BOOTBLOCK_X86_32
-	select ARCH_VERSTAGE_X86_32
-	select ARCH_ROMSTAGE_X86_32
-	select ARCH_RAMSTAGE_X86_32
 	select SMP
 	select UDELAY_TSC
 	select TSC_MONOTONIC_TIMER
 	select UNKNOWN_TSC_RATE
 	select SMM_ASEG
+
+if CPU_QEMU_X86
+
+config CPU_QEMU_X86_64
+	bool "Experimental 64bit support"
+	select ARCH_BOOTBLOCK_X86_64
+	select ARCH_VERSTAGE_X86_64
+	select ARCH_ROMSTAGE_X86_64
+	select ARCH_POSTCAR_X86_64
+	select ARCH_RAMSTAGE_X86_64
+
+config CPU_QEMU_X86_32
+	bool
+	default n if CPU_QEMU_X86_64
+	default y
+	select ARCH_BOOTBLOCK_X86_32
+	select ARCH_VERSTAGE_X86_32
+	select ARCH_ROMSTAGE_X86_32
+	select ARCH_POSTCAR_X86_32
+	select ARCH_RAMSTAGE_X86_32
+endif