ARC: LIB: add ARC MWDT libc support

ARC MWDT toolchain doesn't provide newlib, let's add support
of ARC MWDT libc to Zephyr

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>

lib/libc/CMakeLists.txt

@@ -2,6 +2,8 @@
 
 if(CONFIG_NEWLIB_LIBC)
   add_subdirectory(newlib)
+elseif(CONFIG_ARCMWDT_LIBC)
+  add_subdirectory(arcmwdt)
 else()
   add_subdirectory(minimal)
 endif()

lib/libc/Kconfig

@@ -31,6 +31,13 @@ config NEWLIB_LIBC
 	  Build with newlib library. The newlib library is expected to be
 	  part of the SDK in this case.
 
+config ARCMWDT_LIBC
+	bool "ARC MWDT C library"
+	depends on !NATIVE_APPLICATION
+	depends on "$(ZEPHYR_TOOLCHAIN_VARIANT)" = "arcmwdt"
+	help
+	  C library provided by ARC MWDT toolchain.
+
 config EXTERNAL_LIBC
 	bool "External C library"
 	help

lib/libc/arcmwdt/CMakeLists.txt

@@ -0,0 +1,6 @@
+# SPDX-License-Identifier: Apache-2.0
+
+zephyr_library()
+zephyr_library_sources(
+  arcmwdt-string.c
+)

lib/libc/arcmwdt/arcmwdt-string.c

@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) 2021 Synopsys.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#define __STDC_WANT_LIB_EXT1__ 1
+
+#include <string.h>
+#include <stdint.h>
+#include <sys/types.h>
+
+size_t strnlen(const char *s, size_t maxlen)
+{
+	return strnlen_s(s, maxlen);
+}