Provide our own newlib libc

Newlib 2.2.0 w/ nan ofeatures, xtensa patches, locking
implementation.

Built from https://github.com/projectgus/newlib-xtensa
branch xtensa-2_2_0-lock
commit daa6ae40cdc8099f54c3e68a586fc1b906169c5a

For more details, see:
https://github.com/SuperHouse/esp-open-rtos/wiki/libc-configuration

Closes #1

libc/xtensa-lx106-elf/include/signal.h

@@ -0,0 +1,32 @@
+#ifndef _SIGNAL_H_
+#define _SIGNAL_H_
+
+#include "_ansi.h"
+#include <sys/signal.h>
+
+_BEGIN_STD_C
+
+typedef int	sig_atomic_t;		/* Atomic entity type (ANSI) */
+#ifndef _POSIX_SOURCE
+typedef _sig_func_ptr sig_t;		/* BSD naming */
+typedef _sig_func_ptr sighandler_t;	/* glibc naming */
+#endif /* !_POSIX_SOURCE */
+
+#define SIG_DFL ((_sig_func_ptr)0)	/* Default action */
+#define SIG_IGN ((_sig_func_ptr)1)	/* Ignore action */
+#define SIG_ERR ((_sig_func_ptr)-1)	/* Error return */
+
+struct _reent;
+
+_sig_func_ptr _EXFUN(_signal_r, (struct _reent *, int, _sig_func_ptr));
+int	_EXFUN(_raise_r, (struct _reent *, int));
+
+#ifndef _REENT_ONLY
+_sig_func_ptr _EXFUN(signal, (int, _sig_func_ptr));
+int	_EXFUN(raise, (int));
+void	_EXFUN(psignal, (int, const char *));
+#endif
+
+_END_STD_C
+
+#endif /* _SIGNAL_H_ */