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

cores/esp8266/heap.c

@@ -6,6 +6,27 @@
 #include <stdlib.h>
 #include "umm_malloc/umm_malloc.h"
 #include <c_types.h>
+#include <sys/reent.h>
+
+void* _malloc_r(struct _reent* unused, size_t size)
+{
+    return malloc(size);
+}
+
+void _free_r(struct _reent* unused, void* ptr)
+{
+    return free(ptr);
+}
+
+void* _realloc_r(struct _reent* unused, void* ptr, size_t size)
+{
+    return realloc(ptr, size);
+}
+
+void* _calloc_r(struct _reent* unused, size_t count, size_t size)
+{
+    return calloc(count, size);
+}
 
 void* ICACHE_RAM_ATTR pvPortMalloc(size_t size, const char* file, int line)
 {