LoongArch: Only allow OBJTOOL & ORC unwinder if toolchain supports -mthin-add-sub

GAS <= 2.41 does not support generating R_LARCH_{32,64}_PCREL for
"label - ." and it generates R_LARCH_{ADD,SUB}{32,64} pairs instead.
Objtool cannot handle R_LARCH_{ADD,SUB}{32,64} pair in __jump_table
(static key implementation) and etc. so it will produce some warnings.
This is causing the kernel CI systems to complain everywhere.

For GAS we can check if -mthin-add-sub option is available to know if
R_LARCH_{32,64}_PCREL are supported.

For Clang, we require Clang >= 18 and Clang >= 17 already supports
R_LARCH_{32,64}_PCREL. But unfortunately Clang has some other issues,
so we disable objtool for Clang at present.

Note that __jump_table here is not generated by the compiler, so
-fno-jump-table is completely irrelevant for this issue.

Fixes: cb8a2ef0 ("LoongArch: Add ORC stack unwinder support")
Closes: https://lore.kernel.org/loongarch/Zl5m1ZlVmGKitAof@yujie-X299/
Closes: https://lore.kernel.org/loongarch/ZlY1gDDPi_mNrwJ1@slm.duckdns.org/
Closes: https://lore.kernel.org/loongarch/1717478006.038663-1-hengqi@linux.alibaba.com/
Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=816029e06768
Link: https://github.com/llvm/llvm-project/commit/42cb3c6346fcSigned-off-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/Kconfig

@@ -143,7 +143,7 @@ config LOONGARCH
 	select HAVE_LIVEPATCH
 	select HAVE_MOD_ARCH_SPECIFIC
 	select HAVE_NMI
-	select HAVE_OBJTOOL if AS_HAS_EXPLICIT_RELOCS
+	select HAVE_OBJTOOL if AS_HAS_EXPLICIT_RELOCS && AS_HAS_THIN_ADD_SUB && !CC_IS_CLANG
 	select HAVE_PCI
 	select HAVE_PERF_EVENTS
 	select HAVE_PERF_REGS
@@ -261,6 +261,9 @@ config AS_HAS_EXPLICIT_RELOCS
 config AS_HAS_FCSR_CLASS
 	def_bool $(as-instr,movfcsr2gr \$t0$(comma)\$fcsr0)
 
+config AS_HAS_THIN_ADD_SUB
+	def_bool $(cc-option,-Wa$(comma)-mthin-add-sub)
+
 config AS_HAS_LSX_EXTENSION
 	def_bool $(as-instr,vld \$vr0$(comma)\$a0$(comma)0)
 

arch/loongarch/Kconfig.debug

@@ -28,6 +28,7 @@ config UNWINDER_PROLOGUE
 
 config UNWINDER_ORC
 	bool "ORC unwinder"
+	depends on HAVE_OBJTOOL
 	select OBJTOOL
 	help
 	  This option enables the ORC (Oops Rewind Capability) unwinder for