Fix the comment about which xmm registers to save in w64
Rev: nettle/x86_64/README:1.4
%r15
Additional arguments are passed on the stack. "backing store" on the
-stack for the four register arguments is also required. %xmm6 and
-%xmm7 are callee-saved. The "long" type is just 32 bits. Nettle
+stack for the four register arguments is also required. %xmm6 to
+%xmm15 are callee-saved. The "long" type is just 32 bits. Nettle
currently does *NOT* support this ABI.