Updated x86_64 README.

x86_64/README

@@ -49,8 +49,7 @@ Registers	May be		Argument
 
 Additional arguments are passed on the stack. "backing store" on the
 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.
+%xmm15 are callee-saved. The "long" type is just 32 bits.
 
 If we have five arguments, and push the additional callee-save
 registers %rdi and %rsi on the stack, we get a stack frame like