qemu-system-riscv32 doesn't run BBL/Linux

[mickflemm]

When compiling a 32bit BBL and use it with the -bios parameter on the virt machine, I get an assert here: https://github.com/riscv/riscv-pk/blob/master/machine/minit.c#L78 If I comment this out and try to boot a default 32bit Linux kernel, qemu crashes with "Unable to handle guest executing from RAM within a small MPU region at ...".