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-arm/syscalls/__sys_clone.S b/libc/arch-arm/syscalls/__sys_clone.S
new file mode 100644
index 0000000..9fe2641
--- /dev/null
+++ b/libc/arch-arm/syscalls/__sys_clone.S
@@ -0,0 +1,21 @@
+/* autogenerated by gensyscalls.py */
+#include <sys/linux-syscalls.h>
+
+    .text
+    .type __sys_clone, #function
+    .globl __sys_clone
+    .align 4
+    .fnstart
+
+__sys_clone:
+    mov     ip, sp
+    .save   {r4, r5, r6, r7}
+    stmfd   sp!, {r4, r5, r6, r7}
+    ldmfd   ip, {r4, r5, r6}
+    ldr     r7, =__NR_clone
+    swi     #0
+    ldmfd   sp!, {r4, r5, r6, r7}
+    movs    r0, r0
+    bxpl    lr
+    b       __set_syscall_errno
+    .fnend