kernel: fix ftrace support on 4.9

When porting the kernel patches from 4.4 to 4.9, they were missing a
small chunk that ensures that ftrace sections are kept in the vmlinux
image, even when linked with --gc-sections

Signed-off-by: Felix Fietkau <nbd@nbd.name>

target/linux/generic/hack-4.9/220-gc_sections.patch

@@ -201,6 +201,33 @@ Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
  
 --- a/include/asm-generic/vmlinux.lds.h
 +++ b/include/asm-generic/vmlinux.lds.h
+@@ -105,7 +105,7 @@
+ #ifdef CONFIG_FTRACE_MCOUNT_RECORD
+ #define MCOUNT_REC()	. = ALIGN(8);				\
+ 			VMLINUX_SYMBOL(__start_mcount_loc) = .; \
+-			*(__mcount_loc)				\
++			KEEP(*(__mcount_loc))			\
+ 			VMLINUX_SYMBOL(__stop_mcount_loc) = .;
+ #else
+ #define MCOUNT_REC()
+@@ -113,7 +113,7 @@
+ 
+ #ifdef CONFIG_TRACE_BRANCH_PROFILING
+ #define LIKELY_PROFILE()	VMLINUX_SYMBOL(__start_annotated_branch_profile) = .; \
+-				*(_ftrace_annotated_branch)			      \
++				KEEP(*(_ftrace_annotated_branch))		      \
+ 				VMLINUX_SYMBOL(__stop_annotated_branch_profile) = .;
+ #else
+ #define LIKELY_PROFILE()
+@@ -121,7 +121,7 @@
+ 
+ #ifdef CONFIG_PROFILE_ALL_BRANCHES
+ #define BRANCH_PROFILE()	VMLINUX_SYMBOL(__start_branch_profile) = .;   \
+-				*(_ftrace_branch)			      \
++				KEEP(*(_ftrace_branch))			      \
+ 				VMLINUX_SYMBOL(__stop_branch_profile) = .;
+ #else
+ #define BRANCH_PROFILE()
 @@ -130,7 +130,7 @@
  #ifdef CONFIG_KPROBES
  #define KPROBE_BLACKLIST()	. = ALIGN(8);				      \