lib: Remove executable stack comming from sha1_ssse3.S

Executable stack can be security hole, EX stack prevents working of NX.
More information can be found here:
https://wiki.gentoo.org/wiki/Project:Hardened/GNU_stack_quickstart
https://wiki.gentoo.org/wiki/Hardened/PaX_Utilities#Introduction

i686 architecture wasn't affected.

Signed-off-by: Marcin Mirosław <marcin@mejor.pl>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>