cmd/internal/obj/loong64: save LR after decrementing SP

Refer to CL 413428 and 412474, for loong64, like mips, s390x and riscv, there
is no single instruction that saves the LR and decrements the SP, so we also
need to insert an instruction to save the LR after decrementing the SP.

Fixes golang#56623.
Updates golang#53374.

Change-Id: I3de040792f0a041d3d2a98ea89c23a2dd2f4ad10

Author: abner-chenc

src/cmd/asm/internal/asm/testdata/loong64.s

@@ -5,7 +5,7 @@
 #include "../../../../../runtime/textflag.h"
 // TODO: cover more instruction
 
-TEXT foo(SB),DUPOK|NOSPLIT,$0
-	JAL	1(PC)	//CALL 1(PC)	//000c0054
+TEXT foo(SB),DUPOK|NOSPLIT|NOFRAME,$0
+	JAL	1(PC)	//CALL 1(PC)	//00040054
 	JAL	(R4)	//CALL (R4)	//8100004c
-	JAL	foo(SB)	//CALL foo(SB)	//00100054
+	JAL	foo(SB)	//CALL foo(SB)	//00080054

src/cmd/internal/obj/loong64/obj.go

@@ -260,6 +260,20 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 				q.Spadj = +autosize
 
 				q = c.ctxt.EndUnsafePoint(q, c.newprog, -1)
+
+				// On Linux, in a cgo binary we may get a SIGSETXID signal early on
+				// before the signal stack is set, as glibc doesn't allow us to block
+				// SIGSETXID. So a signal may land on the current stack and clobber
+				// the content below the SP. We store the LR again after the SP is
+				// decremented.
+				q = obj.Appendp(q, newprog)
+				q.As = mov
+				q.Pos = p.Pos
+				q.From.Type = obj.TYPE_REG
+				q.From.Reg = REGLINK
+				q.To.Type = obj.TYPE_MEM
+				q.To.Offset = 0
+				q.To.Reg = REGSP
 			}
 
 			if c.cursym.Func().Text.From.Sym.Wrapper() && c.cursym.Func().Text.Mark&LEAF == 0 {