x86/speculation, objtool: Use absolute relocations for annotations

.discard.retpoline_safe sections do not have the SHF_ALLOC flag.  These
sections referencing text sections' STT_SECTION symbols with PC-relative
relocations like R_386_PC32 [0] is conceptually not suitable.  Newer
LLD will report warnings for REL relocations even for relocatable links [1]:

    ld.lld: warning: vmlinux.a(drivers/i2c/busses/i2c-i801.o):(.discard.retpoline_safe+0x120): has non-ABS relocation R_386_PC32 against symbol ''

Switch to absolute relocations instead, which indicate link-time
addresses.  In a relocatable link, these addresses are also output
section offsets, used by checks in tools/objtool/check.c.  When linking
vmlinux, these .discard.* sections will be discarded, therefore it is
not a problem that R_X86_64_32 cannot represent a kernel address.

Alternatively, we could set the SHF_ALLOC flag for .discard.* sections,
but I think non-SHF_ALLOC for sections to be discarded makes more sense.

Note: if we decide to never support REL architectures (e.g. arm, i386),
we can utilize R_*_NONE relocations (.reloc ., BFD_RELOC_NONE, sym),
making .discard.* sections zero-sized.  That said, the section content
waste is 4 bytes per entry, much smaller than sizeof(Elf{32,64}_Rel).

  [0] commit 1c0c1faf ("objtool: Use relative pointers for annotations")
  [1] https://github.com/ClangBuiltLinux/linux/issues/1937



Signed-off-by: Fangrui Song <maskray@google.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20230920001728.1439947-1-maskray@google.com