qemu: disable qemu user emulation on MIPS64 for host variant

According to target/mips/TODO in the Qemu sources:

MIPS64
------
- Userland emulation (both n32 and n64) not functional.

And indeed, trying to run a mips64n32 binary under qemu user emulation
results in:

Invalid ELF image for this architecture

So we move the BR2_mips64(el) dependency from
BR2_PACKAGE_HOST_QEMU_ARCH_SUPPORTS to
BR2_PACKAGE_HOST_QEMU_SYSTEM_ARCH_SUPPORTS, so that only the system
emulation is available on mips64, and not the user-mode emulation.

Signed-off-by: Adam Duskett <Adamduskett@outlook.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 17024f5900b9803fc34388c39915f70ce51fb037)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/qemu/Config.in.host

@@ -9,8 +9,6 @@ config BR2_PACKAGE_HOST_QEMU_ARCH_SUPPORTS
 	default y if BR2_microblazebe
 	default y if BR2_mips
 	default y if BR2_mipsel
-	default y if BR2_mips64
-	default y if BR2_mips64el
 	default y if BR2_powerpc
 	default y if BR2_powerpc64
 	default y if BR2_powerpc64le
@@ -22,6 +20,8 @@ config BR2_PACKAGE_HOST_QEMU_ARCH_SUPPORTS
 config BR2_PACKAGE_HOST_QEMU_SYSTEM_ARCH_SUPPORTS
 	bool
 	default y if BR2_PACKAGE_HOST_QEMU_ARCH_SUPPORTS
+	default y if BR2_mips64
+	default y if BR2_mips64el
 
 config BR2_PACKAGE_HOST_QEMU_USER_ARCH_SUPPORTS
 	bool