RISC-V: preserve a1 in mcount

The RISC-V ELF psABI states that both a0 and a1 are used for return
values so we should preserve them both in return_to_handler.  This is
especially important for RV32 for functions returning a 64-bit quantity
otherwise the return value can be corrupted and undefined behaviour
results.

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Link: https://lore.kernel.org/r/20221115200832.706370-4-jamie@jamieiles.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/kernel/mcount.S

@@ -29,6 +29,7 @@
 	REG_S	s0, 2*SZREG(sp)
 	REG_S	ra, 3*SZREG(sp)
 	REG_S	a0, 1*SZREG(sp)
+	REG_S	a1, 0*SZREG(sp)
 	addi	s0, sp, 4*SZREG
 	.endm
 
@@ -42,6 +43,7 @@
 	REG_L	ra, 3*SZREG(sp)
 	REG_L	s0, 2*SZREG(sp)
 	REG_L	a0, 1*SZREG(sp)
+	REG_L	a1, 0*SZREG(sp)
 	addi	sp, sp, 4*SZREG
 	.endm
 
@@ -71,9 +73,9 @@ ENTRY(return_to_handler)
 	mv	a0, t6
 #endif
 	call	ftrace_return_to_handler
-	mv	a1, a0
+	mv	a2, a0
 	RESTORE_RET_ABI_STATE
-	jalr	a1
+	jalr	a2
 ENDPROC(return_to_handler)
 #endif