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/alloca.h

@@ -0,0 +1,21 @@
+/* libc/include/alloca.h - Allocate memory on stack */
+
+/* Written 2000 by Werner Almesberger */
+/* Rearranged for general inclusion by stdlib.h.
+   2001, Corinna Vinschen <vinschen@redhat.com> */
+
+#ifndef _NEWLIB_ALLOCA_H
+#define _NEWLIB_ALLOCA_H
+
+#include "_ansi.h"
+#include <sys/reent.h>
+
+#undef alloca
+
+#ifdef __GNUC__
+#define alloca(size) __builtin_alloca(size)
+#else
+void * _EXFUN(alloca,(size_t));
+#endif
+
+#endif