metag: add missing header file

When installing header files a warning is generated, because
the header is mandatory.

libc/sysdeps/linux/metag/jmpbuf-unwind.h

@@ -0,0 +1,22 @@
+/*
+ * Licensed under the LGPL v2.1, see the file COPYING.LIB in this tarball.
+ */
+
+#include <setjmp.h>
+
+/* Test if longjmp to JMPBUF would unwind the frame
+   containing a local variable at ADDRESS.  */
+#undef _JMPBUF_UNWINDS
+#define _JMPBUF_UNWINDS(jmpbuf, address, demangle) \
+  ((void *) (address) > (void *) demangle (jmpbuf[JB_SP]))
+
+#ifdef __UCLIBC_HAS_THREADS_NATIVE__
+#include <stdint.h>
+#include <unwind.h>
+
+#define _JMPBUF_CFA_UNWINDS_ADJ(_jmpbuf, _context, _adj) \
+  _JMPBUF_UNWINDS_ADJ (_jmpbuf, (void *) _Unwind_GetCFA (_context), _adj)
+
+#define _JMPBUF_UNWINDS_ADJ(_jmpbuf, _address, _adj) \
+  ((uintptr_t) (_address) - (_adj) > (uintptr_t) (_jmpbuf)[JB_SP] - (_adj))
+#endif