Allow ABIs where SP points below the stack frame.
Allow ABIs where SP points below the stack frame.

On C6X, the stack pointer points to a word that is not part of the current
function's stack frame.  It may be overwritten by callees.  Take this into
account when creating the stack for a cloned thread.

Signed-off-by: Bernd Schmidt <c931ab6ac2d0c45d944fa77d0e13e53c91dbbf4d@codesourcery.com>

joel-porquet/tsar-uclibc,joel-porquet/tsar-uclibc,joel-porquet/tsar-uclibc,joel-porquet/tsar-uclibc