Use only unsigned arithmetic in constant-time operations

Signed-off-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Emilia Käsper <emilia@openssl.org>
md32_common.h: address compiler warning in HOST_c2l.

Reviewed-by: Stephen Henson <steve@openssl.org>
(cherry picked from commit d45282fc7cd9b97ed1479f8b8af713337fce57f5)
Merge branch 'OpenSSL_1_0_1-stable' into 1.0.1-chacha