riscv: support standalone applications on RV64I systems

Add an implementation of EXPORT_FUNC() for RV64I systems to support them
in standalone applications.

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

@@ -176,6 +176,16 @@ gd_t *global_data;
 /*
  * gp holds the pointer to the global_data. t0 is call clobbered.
  */
+#ifdef CONFIG_ARCH_RV64I
+#define EXPORT_FUNC(f, a, x, ...)	\
+	asm volatile (			\
+"	.globl " #x "\n"		\
+#x ":\n"				\
+"	ld	t0, %0(gp)\n"		\
+"	ld	t0, %1(t0)\n"		\
+"	jr	t0\n"			\
+	: : "i"(offsetof(gd_t, jt)), "i"(FO(x)) : "t0");
+#else
 #define EXPORT_FUNC(f, a, x, ...)	\
 	asm volatile (			\
 "	.globl " #x "\n"		\
@@ -184,6 +194,7 @@ gd_t *global_data;
 "	lw	t0, %1(t0)\n"		\
 "	jr	t0\n"			\
 	: : "i"(offsetof(gd_t, jt)), "i"(FO(x)) : "t0");
+#endif
 #elif defined(CONFIG_ARC)
 /*
  * r25 holds the pointer to the global_data. r10 is call clobbered.