LoongArch: Only use SC.Q when supported by the assembler

The 128-bit atomic cmpxchg implementation uses the SC.Q instruction.
Older versions of GNU AS do not support that instruction, erroring out:

ERROR:root:{standard input}: Assembler messages:
{standard input}:4831: Error: no match insn: sc.q	$t0,$t1,$r14
{standard input}:6407: Error: no match insn: sc.q	$t0,$t1,$r23
{standard input}:10856: Error: no match insn: sc.q	$t0,$t1,$r14

make[4]: *** [../scripts/Makefile.build:289: mm/slub.o] Error 1

(Binutils 2.41)

So test support for SC.Q in Kconfig and disable the atomics if the
instruction is not available.

Fixes: f0e4b1b ("LoongArch: Add 128-bit atomic cmpxchg support")
Closes: https://lore.kernel.org/lkml/20260216082834-edc51c46-7b7a-4295-8ea5-4d9a3ca2224f@linutronix.de/
Reviewed-by: Xi Ruoyao <xry111@xry111.site>
Acked-by: Hengqi Chen <hengqi.chen@gmail.com>
Tested-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

Author: t-8ch

arch/loongarch/Kconfig

@@ -304,6 +304,9 @@ config AS_HAS_LBT_EXTENSION
 config AS_HAS_LVZ_EXTENSION
 	def_bool $(as-instr,hvcl 0)
 
+config AS_HAS_SCQ_EXTENSION
+	def_bool $(as-instr,sc.q \$t0$(comma)\$t1$(comma)\$t2)
+
 config CC_HAS_ANNOTATE_TABLEJUMP
 	def_bool $(cc-option,-mannotate-tablejump)
 

arch/loongarch/include/asm/cmpxchg.h

@@ -238,6 +238,8 @@ __cmpxchg(volatile void *ptr, unsigned long old, unsigned long new, unsigned int
 	arch_cmpxchg((ptr), (o), (n));					\
   })
 
+#ifdef CONFIG_AS_HAS_SCQ_EXTENSION
+
 union __u128_halves {
 	u128 full;
 	struct {
@@ -290,6 +292,9 @@ union __u128_halves {
 	BUILD_BUG_ON(sizeof(*(ptr)) != 16);				\
 	__arch_cmpxchg128(ptr, o, n, "");				\
 })
+
+#endif /* CONFIG_AS_HAS_SCQ_EXTENSION */
+
 #else
 #include <asm-generic/cmpxchg-local.h>
 #define arch_cmpxchg64_local(ptr, o, n) __generic_cmpxchg64_local((ptr), (o), (n))