minios: implement ffs, ffsl and ffsll.

minios: implement ffs, ffsl and ffsll.

The first function is compiled only in case minios is compiled without
newlib, since newlib already provides an implementation for ffs.
On the other hand ffsl and ffsll are always compiled because newlib
misses those functions.
This patch also provides an implementation for __ffsti2 and __ffsdi2
because they are needed by gcc in order to successfully link ffsll.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>