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

+#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));
+}

src/thread/arm/clone.s

 .text
 .global __clone
-.weak clone
 .type   __clone,%function
-.type   clone,%function
 __clone:
-clone:
 	stmfd sp!,{r4,r5,r6,r7}
 	mov r7,#120
 	mov r6,r3

src/thread/clone.c

 #include <errno.h>
-#include "libc.h"
+#include "pthread_impl.h"
 
 int __clone(int (*func)(void *), void *stack, int flags, void *arg, ...)
 {
-	errno = ENOSYS;
-	return -1;
+	return -ENOSYS;
 }
-
-weak_alias(__clone, clone);

src/thread/i386/clone.s

 .text
 .global __clone
-.weak clone
 .type   __clone,@function
-.type   clone,@function
 __clone:
-clone:
 	push %ebp
 	mov %esp,%ebp
 	push %ebx

src/thread/microblaze/clone.s

 .global __clone
-.weak clone
 .type   __clone,@function
-.type   clone,@function
 
 # r5, r6, r7, r8, r9, r10, stack
 # fn, st, fl, ar, pt, tl, ct
 # fl, st, __, pt, ct, tl
 
 __clone:
-clone:
 	andi    r6, r6, -16
 	addi    r6, r6, -16
 	swi     r5, r6, 0

src/thread/x86_64/clone.s

 .text
 .global __clone
-.weak clone
 .type   __clone,@function
-.type   clone,@function
 __clone:
-clone:
 	xor %eax,%eax
 	mov $56,%al
 	mov %rdi,%r11