crypto: vmx - various build fixes

First up, clean up the generated .S files properly on a 'make clean'.
Secondly, force re-generation of these files when building for different
endian-ness than what was built previously. Finally, generate the new
files in the build tree, rather than the source tree.

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>