Makefile: Remove -mno-direct-move cflag

GCC 8 warns that -mno-direct-move is depreciated. We had it there so
we wouldn't use VSX registers in skiboot, as they are not
saved/restored, however Segher confirms:

 > if you already have -mno-altivec  then -mno-direct-move does zilch

So it was never doing anything.

Resolves: #186
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Stewart Smith <stewart@linux.ibm.com>

Makefile.main

@@ -179,7 +179,6 @@ CFLAGS += $(call try-cflag,$(CC),-mno-multiple)
 # do not use any automatic vector foo
 # While it would be safe during boot, we don't save/restore across OPAL calls
 CFLAGS += $(call try-cflag,$(CC),-mno-vsx) \
-	$(call try-cflag,$(CC),-mno-direct-move) \
 	$(call try-cflag,$(CC),-mno-altivec)
 
 # Do not use load/store update. You REALLY do not want to use this!