lkdtm: Fix content of section containing lkdtm_rodata_do_nothing()

On a kernel without CONFIG_STRICT_KERNEL_RWX, running EXEC_RODATA
test leads to "Illegal instruction" failure.

Looking at the content of rodata_objcopy.o, we see that the
function content zeroes only:

Disassembly of section .rodata:

0000000000000000 <.lkdtm_rodata_do_nothing>:
0: 00 00 00 00 .long 0x0

Add the contents flag in order to keep the content of the section
while renaming it.

Disassembly of section .rodata:

0000000000000000 <.lkdtm_rodata_do_nothing>:
0: 4e 80 00 20 blr

Fixes: e9e08a07385e ("lkdtm: support llvm-objcopy")
Cc: [e-mail]
Cc: Kees Cook <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Nick Desaulniers <[e-mail]>
Cc: Nathan Chancellor <[e-mail]>
Signed-off-by: Christophe Leroy <[e-mail]>
Reviewed-by: Nick Desaulniers <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Link: https://lore.kernel.org/r/8900731f[e-mail]

drivers/misc/lkdtm/Makefile

···
 
 OBJCOPYFLAGS :=
 OBJCOPYFLAGS_rodata_objcopy.o	:= \
-			--rename-section .noinstr.text=.rodata,alloc,readonly,load
+			--rename-section .noinstr.text=.rodata,alloc,readonly,load,contents
 targets += rodata.o rodata_objcopy.o
 $(obj)/rodata_objcopy.o: $(obj)/rodata.o FORCE
 	$(call if_changed,objcopy)