qemu: introduce BR2_PACKAGE_HOST_QEMU_{SYSTEM, USER}_ARCH_SUPPORTS

qemu: introduce BR2_PACKAGE_HOST_QEMU_{SYSTEM, USER}_ARCH_SUPPORTS

Not all architectures are supported by both the system emulation and
user-mode emulation in Qemu, so a single
BR2_PACKAGE_HOST_QEMU_ARCH_SUPPORTS doesn't work very well.

Therefore, this commit introduces the
BR2_PACKAGE_HOST_QEMU_{SYSTEM,USER}_ARCH_SUPPORTS hidden options. We
keep the BR2_PACKAGE_HOST_QEMU_ARCH_SUPPORTS option for the (numerous)
architectures supported by both system emulation and user-mode
emulation.

The 'select' logic to make sure that at least either system emulation
or user-mode emulation is selected is reworked, and done carefully to
avoid recursive Kconfig dependencies.

For now BR2_PACKAGE_HOST_QEMU_SYSTEM_ARCH_SUPPORTS and
BR2_PACKAGE_HOST_QEMU_USER_ARCH_SUPPORTS are the same, but they will
become different in a follow-up commit.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>