Use libc from newlib (#1752)

* Use newlib libc library

This change adds libcmin.a, which is created from newlib libc by selectively removing some of the object files (mostly related to heap management).
The list of files is available in tools/sdk/lib/make_libcmin.sh. Files which are not needed are commented out.
This change adds support for various functions which were missing, like sscanf, strftime, etc.

* Fix some of the time functions

* Redirect stdout to serial

* Implement __putc_r

* Switch to custom newlib build

Built from https://github.com/igrr/newlib-xtensa using:
./configure --with-newlib --enable-multilib --disable-newlib-io-c99-formats --enable-newlib-supplied-syscalls --enable-target-optspace --program-transform-name="s&^&xtensa-lx106-elf-&" --disable-option-checking --with-target-subdir=xtensa-lx106-elf --target=xtensa-lx106-elf --enable-newlib-nano-formatted-io --enable-newlib-reent-small  --prefix=path-to-arduino-core/tools/sdk/libc
CROSS_CFLAGS="-DMALLOC_PROVIDED -DSIGNAL_PROVIDED -DABORT_PROVIDED" make
make install

* Update tests

tools/sdk/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_ */