examples: implement _start and syscall for RISC-V

To build the API examples on RISC-V we need to
implement _start and syscall for RISC-V.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

examples/api/crt0.S

@@ -80,6 +80,38 @@ syscall:
 return_addr:
 	.align 8
 	.long 0
+#elif defined(CONFIG_ARCH_RV32I)
+
+	.text
+	.globl _start
+_start:
+	la	t0, search_hint
+	sw	sp, 0(t0)
+	la	t0, main
+	jalr	x0, t0
+
+	.globl syscall
+syscall:
+	la	t0, syscall_ptr
+	lw	t0, 0(t0)
+	jalr	x0, t0
+
+#elif defined(CONFIG_ARCH_RV64I)
+
+	.text
+	.globl _start
+_start:
+	la	t0, search_hint
+	sd	sp, 0(t0)
+	la	t0, main
+	jalr	x0, t0
+
+	.globl syscall
+syscall:
+	la	t0, syscall_ptr
+	ld	t0, 0(t0)
+	jalr	x0, t0
+
 #else
 #error No support for this arch!
 #endif