sh: don't allow non-coherent DMA for NOMMU

The code handling non-coherent DMA depends on being able to remap code
as non-cached.  But that can't be done without an MMU, so using this
option on NOMMU builds is broken.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rich Felker <dalias@libc.org>