nds32: Fix gcc 8.0 compiler option incompatible.

When the kernel configs of ftrace and frame pointer options are
choosed, the compiler option of kernel will incompatible.
	Error message:
		nds32le-linux-gcc: error: -pg and -fomit-frame-pointer are incompatible
Signed-off-by: Nickhu <nickhu@andestech.com>
Signed-off-by: Zong Li <zong@andestech.com>
Acked-by: Greentime Hu <greentime@andestech.com>
Signed-off-by: Greentime Hu <greentime@andestech.com>

arch/nds32/mm/Makefile

@@ -4,4 +4,8 @@ obj-y				:= extable.o tlb.o \
 
 obj-$(CONFIG_ALIGNMENT_TRAP)	+= alignment.o
 obj-$(CONFIG_HIGHMEM)           += highmem.o
-CFLAGS_proc-n13.o		+= -fomit-frame-pointer
+
+ifdef CONFIG_FUNCTION_TRACER
+CFLAGS_REMOVE_proc.o     = $(CC_FLAGS_FTRACE)
+endif
+CFLAGS_proc.o              += -fomit-frame-pointer