riscv: replace use of callee-saved register in standalone

Register x19 (s3) is a callee-saved register. It must not be used to
load and jump to exported functions without saving it beforehand.
Replace it with t0, a temporary and caller-saved register.

Change the code comment to reflect this and fix it to correctly list gp
as the register with the pointer to global data.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>

examples/standalone/stubs.c

@@ -174,16 +174,16 @@ gd_t *global_data;
 	: : "i"(offsetof(gd_t, jt)), "i"(FO(x)) : "$r16");
 #elif defined(CONFIG_RISCV)
 /*
- * t7 holds the pointer to the global_data. gp is call clobbered.
+ * gp holds the pointer to the global_data. t0 is call clobbered.
  */
 #define EXPORT_FUNC(f, a, x, ...)	\
 	asm volatile (			\
 "	.globl " #x "\n"		\
 #x ":\n"				\
-"	lw	x19, %0(gp)\n"		\
-"	lw	x19, %1(x19)\n"		\
-"	jr	x19\n"			\
-	: : "i"(offsetof(gd_t, jt)), "i"(FO(x)) : "x19");
+"	lw	t0, %0(gp)\n"		\
+"	lw	t0, %1(t0)\n"		\
+"	jr	t0\n"			\
+	: : "i"(offsetof(gd_t, jt)), "i"(FO(x)) : "t0");
 #elif defined(CONFIG_ARC)
 /*
  * r25 holds the pointer to the global_data. r10 is call clobbered.