LoongArch: Make the users of larch_insn_gen_break() constant

LoongArch defines UPROBE_SWBP_INSN as a function call and this breaks
arch_uprobe_trampoline() which uses it to initialize a static variable.

Add the new "__builtin_constant_p" helper, __emit_break(), and redefine
the current users of larch_insn_gen_break() to use it.

Fixes: ff474a78 ("uprobe: Add uretprobe syscall to speed up return probe")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://lore.kernel.org/all/20240614174822.GA1185149@thelio-3990X/Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/include/asm/inst.h

@@ -532,6 +532,9 @@ static inline void emit_##NAME(union loongarch_instruction *insn,	\
 
 DEF_EMIT_REG0I15_FORMAT(break, break_op)
 
+/* like emit_break(imm) but returns a constant expression */
+#define __emit_break(imm)	((u32)((imm) | (break_op << 15)))
+
 #define DEF_EMIT_REG0I26_FORMAT(NAME, OP)				\
 static inline void emit_##NAME(union loongarch_instruction *insn,	\
 			       int offset)				\

arch/loongarch/include/asm/uprobes.h

@@ -9,10 +9,10 @@ typedef u32 uprobe_opcode_t;
 #define MAX_UINSN_BYTES		8
 #define UPROBE_XOL_SLOT_BYTES	MAX_UINSN_BYTES
 
-#define UPROBE_SWBP_INSN	larch_insn_gen_break(BRK_UPROBE_BP)
+#define UPROBE_SWBP_INSN	__emit_break(BRK_UPROBE_BP)
 #define UPROBE_SWBP_INSN_SIZE	LOONGARCH_INSN_SIZE
 
-#define UPROBE_XOLBP_INSN	larch_insn_gen_break(BRK_UPROBE_XOLBP)
+#define UPROBE_XOLBP_INSN	__emit_break(BRK_UPROBE_XOLBP)
 
 struct arch_uprobe {
 	unsigned long	resume_era;

arch/loongarch/kernel/kprobes.c

@@ -4,8 +4,8 @@
 #include <linux/preempt.h>
 #include <asm/break.h>
 
-#define KPROBE_BP_INSN		larch_insn_gen_break(BRK_KPROBE_BP)
-#define KPROBE_SSTEPBP_INSN	larch_insn_gen_break(BRK_KPROBE_SSTEPBP)
+#define KPROBE_BP_INSN		__emit_break(BRK_KPROBE_BP)
+#define KPROBE_SSTEPBP_INSN	__emit_break(BRK_KPROBE_SSTEPBP)
 
 DEFINE_PER_CPU(struct kprobe *, current_kprobe);
 DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);