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

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/20240924062710.1243-1-yangtiezhu@loongson.cn
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: <stable@vger.kernel.org>	[6.9+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>