clone: make clone a wrapper around __clone

The architecture-specific assembly versions of clone did not set errno on
failure, which is inconsistent with glibc.  __clone still returns the error
via its return value, and clone is now a wrapper that sets errno as needed.
The public clone has also been moved to src/linux, as it's not directly
related to the pthreads API.

__clone is called by pthread_create, which does not report errors via
errno.  Though not strictly necessary, it's nice to avoid clobbering errno
here.

src/linux/clone.c

@@ -0,0 +1,19 @@
+#include <stdarg.h>
+#include <unistd.h>
+#include "pthread_impl.h"
+#include "syscall.h"
+
+int clone(int (*func)(void *), void *stack, int flags, void *arg, ...)
+{
+	va_list ap;
+	pid_t *ptid, *ctid;
+	void  *tls;
+
+	va_start(ap, arg);
+	ptid = va_arg(ap, pid_t *);
+	tls  = va_arg(ap, void *);
+	ctid = va_arg(ap, pid_t *);
+	va_end(ap);
+
+	return __syscall_ret(__clone(func, stack, flags, arg, ptid, tls, ctid));
+}