x86: generic: use HIGHMEM64G instead of HIGHMEM4G

commit 4b4f73937371 switched on HIGHMEM4G which implicitely disabled
PAE and hence also NX and other useful and security-relevant features.
Re-enable PAE by switching to HIGHMEM64G.

Signed-off-by: Daniel Golle <[email protected]>

target/linux/x86/generic/config-default

@@ -175,7 +175,8 @@ CONFIG_HDMI=y
 CONFIG_HIBERNATE_CALLBACKS=y
 CONFIG_HID=y
 CONFIG_HID_BATTERY_STRENGTH=y
-# CONFIG_HIGHMEM64G is not set
+# CONFIG_HIGHMEM4G is not set
+CONFIG_HIGHMEM64G=y
 CONFIG_HOTPLUG_CPU=y
 CONFIG_HPET=y
 CONFIG_HPET_MMAP=y