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/Android.mk b/libc/Android.mk
index c32cc30..e820b60 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -289,6 +289,7 @@
 ifeq ($(TARGET_ARCH),arm)
 libc_common_src_files += \
 	bionic/eabi.c \
+	bionic/bionic_clone.c \
 	arch-arm/bionic/__get_pc.S \
 	arch-arm/bionic/__get_sp.S \
 	arch-arm/bionic/_exit_with_stack_teardown.S \