riscv64: add vfork

Implement vfork() using clone(CLONE_VM | CLONE_VFORK | ...).

src/process/riscv64/vfork.s

@@ -0,0 +1,12 @@
+.global vfork
+.type vfork,@function
+vfork:
+	/* riscv does not have SYS_vfork, so we must use clone instead */
+	/* note: riscv's clone = clone(flags, sp, ptidptr, tls, ctidptr) */
+	li a7, 220
+	li a0, 0x100 | 0x4000 | 17 /* flags = CLONE_VM | CLONE_VFORK | SIGCHLD */
+	mv a1, sp
+	/* the other arguments are ignoreable */
+	ecall
+	.hidden __syscall_ret
+	j __syscall_ret