arch/Config.in: BR2_BINFMT_FLAT remove architecture dependencies

The commit [1] removed the Config.in.host file and moved the arm, sh,
sparc, xtensa and riscv-64 architectures dependencies.  But this list
doesn't contain all elf2flt supported architectures and this breaks
the host-gcc-initial build for m68k.

Since BR2_BINFMT_FLAT is only available on no MMU case, we can rely on
BR2_USE_MMU option and remove the architecture list.

Note: SH without MMU support has been removed by [2], sparc requires
an MMU since at least [3].

Fixes:

  http://autobuild.buildroot.net/results/15d62edfc4b54c0b40b9d3961b797bce4e307919/



[1] 9db5eb25
[2] 04ea3d38
[3] 29563047

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>