objtool: Use R_*_NONE relocations for annotations

.discard.retpoline_safe sections do not have the SHF_ALLOC flag.  These
sections referencing text sections with PC-relative relocations like
R_386_PC32 [0] are conceptually not suitable.  Newer LLD will report
warnings 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 ''

Since the annotation is RELA only (therefore i386 is unsupported), we
can utilitize R_*_NONE relocations for annotation, making
.discard.retpoline_safe sections zero-sized.  If we ever support i386
REL relocations, which requires teaching reloc_addend implicit addends,
we can utilitize .long directives just for REL.

[0]: commit 1c0c1fa ("objtool: Use relative pointers for
annotations")

Link: ClangBuiltLinux#1937 [1]
Signed-off-by: Fangrui Song <maskray@google.com>

arch/x86/include/asm/nospec-branch.h

@@ -196,7 +196,7 @@
 .macro ANNOTATE_RETPOLINE_SAFE
 .Lhere_\@:
 	.pushsection .discard.retpoline_safe
-	.long .Lhere_\@ - .
+	.reloc .,  BFD_RELOC_NONE, .Lhere_\@
 	.popsection
 .endm
 
@@ -334,7 +334,7 @@
 #define ANNOTATE_RETPOLINE_SAFE					\
 	"999:\n\t"						\
 	".pushsection .discard.retpoline_safe\n\t"		\
-	".long 999b - .\n\t"					\
+	".reloc ., BFD_RELOC_NONE, 999b\n\t"			\
 	".popsection\n\t"
 
 typedef u8 retpoline_thunk_t[RETPOLINE_THUNK_SIZE];