Implement clone() C library function properly.

Only provide an implementation for ARM at the moment, since
it requires specific assembly fragments (the standard syscall
stubs cannot be used because the child returns in a different
stack).
diff --git a/libc/arch-sh/syscalls/__clone.S b/libc/arch-sh/syscalls/__sys_clone.S
similarity index 74%
rename from libc/arch-sh/syscalls/__clone.S
rename to libc/arch-sh/syscalls/__sys_clone.S
index 1df6ca2..c2e7dd2 100644
--- a/libc/arch-sh/syscalls/__clone.S
+++ b/libc/arch-sh/syscalls/__sys_clone.S
@@ -2,15 +2,18 @@
 #include <sys/linux-syscalls.h>
 
     .text
-    .type __clone, @function
-    .globl __clone
+    .type __sys_clone, @function
+    .globl __sys_clone
     .align 4
 
-__clone:
+__sys_clone:
+
+    /* get ready for additonal arg */
+    mov.l   @r15, r0
 
     /* invoke trap */
     mov.l   0f, r3  /* trap num */
-    trapa   #(4 + 0x10)
+    trapa   #(5 + 0x10)
 
     /* check return value */
     cmp/pz  r0