x86-64: globally use -mno-sse

This eliminates pointless prologue code from functions having variable
argument lists (since that way xmm registers can't possibly be passed).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/Rules.mk
+++ b/xen/arch/x86/Rules.mk
@@ -41,7 +41,7 @@ x86_64 := n
 endif
 
 ifeq ($(TARGET_SUBARCH),x86_64)
-CFLAGS += -mno-red-zone -fpic
+CFLAGS += -mno-red-zone -mno-sse -fpic
 CFLAGS += -fno-asynchronous-unwind-tables
 # -fvisibility=hidden reduces -fpic cost, if it's available
 ifneq ($(call cc-option,$(CC),-fvisibility=hidden,n),n)
--- a/xen/arch/x86/efi/Makefile
+++ b/xen/arch/x86/efi/Makefile
@@ -1,4 +1,4 @@
-CFLAGS += -fshort-wchar -mno-sse
+CFLAGS += -fshort-wchar
 
 obj-y += stub.o