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/sys/wait.h

@@ -0,0 +1,44 @@
+#ifndef _SYS_WAIT_H
+#define _SYS_WAIT_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <sys/types.h>
+
+#define WNOHANG 1
+#define WUNTRACED 2
+
+/* A status looks like:
+      <2 bytes info> <2 bytes code>
+
+      <code> == 0, child has exited, info is the exit value
+      <code> == 1..7e, child has exited, info is the signal number.
+      <code> == 7f, child has stopped, info was the signal number.
+      <code> == 80, there was a core dump.
+*/
+   
+#define WIFEXITED(w)	(((w) & 0xff) == 0)
+#define WIFSIGNALED(w)	(((w) & 0x7f) > 0 && (((w) & 0x7f) < 0x7f))
+#define WIFSTOPPED(w)	(((w) & 0xff) == 0x7f)
+#define WEXITSTATUS(w)	(((w) >> 8) & 0xff)
+#define WTERMSIG(w)	((w) & 0x7f)
+#define WSTOPSIG	WEXITSTATUS
+
+pid_t wait (int *);
+pid_t waitpid (pid_t, int *, int);
+
+#ifdef _COMPILING_NEWLIB
+pid_t _wait (int *);
+#endif
+
+/* Provide prototypes for most of the _<systemcall> names that are
+   provided in newlib for some compilers.  */
+pid_t _wait (int *);
+
+#ifdef __cplusplus
+};
+#endif
+
+#endif