Add stackmarkings to stream_salsa20_amd64_xmm6.s

I'm trying to package libsodium for gentoo and received this warning:
 * !WX --- --- usr/lib64/libsodium.a:stream_salsa20_amd64_xmm6.o

According to http://www.gentoo.org/proj/en/hardened/gnu-stack.xml
this commit fixes it.

Please handle this with care, as I am no asm guru.