compiler.h: specify correct attribute for .rodata..c_jump_table

Currently, there is an assembler message when generating kernel/bpf/core.o
under CONFIG_OBJTOOL with LoongArch compiler toolchain:

Warning: setting incorrect section attributes for .rodata..c_jump_table

This is because the section ".rodata..c_jump_table" should be readonly,
but there is a "W" (writable) part of the flags:

$ readelf -S kernel/bpf/core.o | grep -A 1 "rodata..c"
[34] .rodata..c_j[...] PROGBITS 0000000000000000 0000d2e0
0000000000000800 0000000000000000 WA 0 0 8

There is no above issue on x86 due to the generated section flag is only
"A" (allocatable). In order to silence the warning on LoongArch, specify
the attribute like ".rodata..c_jump_table,\"a\",@progbits #" explicitly,
then the section attribute of ".rodata..c_jump_table" must be readonly
in the kernel/bpf/core.o file.

Before:

$ objdump -h kernel/bpf/core.o | grep -A 1 "rodata..c"
21 .rodata..c_jump_table 00000800 0000000000000000 0000000000000000 0000d2e0 2**3
CONTENTS, ALLOC, LOAD, RELOC, DATA

After:

$ objdump -h kernel/bpf/core.o | grep -A 1 "rodata..c"
21 .rodata..c_jump_table 00000800 0000000000000000 0000000000000000 0000d2e0 2**3
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA

By the way, AFAICT, maybe the root cause is related with the different
compiler behavior of various archs, so to some extent this change is a
workaround for LoongArch, and also there is no effect for x86 which is the
only port supported by objtool before LoongArch with this patch.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Tiezhu Yang <[e-mail]>
Cc: Josh Poimboeuf <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: <[e-mail]> [6.9+]
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/compiler.h

···
 #define annotate_unreachable() __annotate_unreachable(__COUNTER__)
 
 /* Annotate a C jump table to allow objtool to follow the code flow */
-#define __annotate_jump_table __section(".rodata..c_jump_table")
+#define __annotate_jump_table __section(".rodata..c_jump_table,\"a\",@progbits #")
 
 #else /* !CONFIG_OBJTOOL */
 #define annotate_reachable()