build: move setting LTO options to xen/Rules.mk

Having them in StdGNU.mk would affect both hypervisor and tools build.
However judging from the commit message of e4cdd74f LTO was only meant
to affect hypvervisor build.

Move the relevant bits to xen/Rules.mk.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

config/StdGNU.mk

@@ -35,7 +35,3 @@ UTIL_LIBS = -lutil
 SONAME_LDFLAG = -soname
 SHLIB_LDFLAGS = -shared
 
-ifeq ($(lto),y)
-CFLAGS += -flto
-LDFLAGS-$(clang) += -plugin LLVMgold.so
-endif

xen/Rules.mk

@@ -120,6 +120,8 @@ $(filter-out %.init.o $(nogcov-y),$(obj-y) $(obj-bin-y) $(extra-y)): CFLAGS += -
 endif
 
 ifeq ($(CONFIG_LTO),y)
+CFLAGS += -flto
+LDFLAGS-$(clang) += -plugin LLVMgold.so
 # Would like to handle all object files as bitcode, but objects made from
 # pure asm are in a different format and have to be collected separately.
 # Mirror the directory tree, collecting them as built_in_bin.o.