riscv: Fixup wrong ftrace remove cflag

We must use $(CC_FLAGS_FTRACE) instead of directly using -pg. It
will cause -fpatchable-function-entry error.
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/kernel/Makefile

@@ -4,8 +4,8 @@
 #
 
 ifdef CONFIG_FTRACE
-CFLAGS_REMOVE_ftrace.o	= -pg
-CFLAGS_REMOVE_patch.o	= -pg
+CFLAGS_REMOVE_ftrace.o	= $(CC_FLAGS_FTRACE)
+CFLAGS_REMOVE_patch.o	= $(CC_FLAGS_FTRACE)
 endif
 
 extra-y += head.o

arch/riscv/mm/Makefile

@@ -2,7 +2,7 @@
 
 CFLAGS_init.o := -mcmodel=medany
 ifdef CONFIG_FTRACE
-CFLAGS_REMOVE_init.o = -pg
+CFLAGS_REMOVE_init.o = $(CC_FLAGS_FTRACE)
 endif
 
 KCOV_INSTRUMENT_init.o := n