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/docs/CHANGES.TXT b/libc/docs/CHANGES.TXT
index 07eac57..c2655c5 100644
--- a/libc/docs/CHANGES.TXT
+++ b/libc/docs/CHANGES.TXT
@@ -46,6 +46,7 @@
 
 - <wchar.h>: Add mbstowcs() and wcstombs()
 
+- add clone() implementation for ARM (x86 and SH-4 not working yet)
 
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