Move log2ceil from riscv_pmp to libc. Also, implement log2floor for
completeness.

These are the run-time alternative to the compile-time macros.

DMA directly to user (virtual) memory won't work, as the DMA engine(s)
don't do address translations, i.e. they require a physical address.

Using kernel heap is fine as it is mapped vaddr=paddr. Also, the USB DMA
engine does not have any alignment requirements.

The hack just opens the entire SoC memory unconditionally, which is not
a good idea.

Test features can be used ad-hoc, they don't need to be supported by the
build.