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/param.h

@@ -0,0 +1,28 @@
+/* This is a dummy <sys/param.h> file, not customized for any
+   particular system.  If there is a param.h in libc/sys/SYSDIR/sys,
+   it will override this one.  */
+
+#ifndef _SYS_PARAM_H
+# define _SYS_PARAM_H
+
+#include <sys/config.h>
+#include <sys/syslimits.h>
+#include <machine/endian.h>
+#include <machine/param.h>
+
+#ifndef HZ
+# define HZ (60)
+#endif
+#ifndef NOFILE
+# define NOFILE	(60)
+#endif
+#ifndef PATHSIZE
+# define PATHSIZE (1024)
+#endif
+
+#define MAXPATHLEN PATH_MAX
+
+#define MAX(a,b) ((a) > (b) ? (a) : (b))
+#define MIN(a,b) ((a) < (b) ? (a) : (b))
+
+#endif