lib/newlib: revert treatment of libc files as system includes

The solution from #14312 of using -isystem to prioritize the position of
the libc directory bypasses the effect of -ffreestanding with respect to
libc symbols expected to be present in a non-hosted environment.

Further, it breaks C++ with the ARM Embedded toolchain as the system
fails to find the right file with #include_next.

Use a more fine-grained solution that explicitly includes the underlying
newlib header required for <inttypes.h> support before moving on to
include the next available one, whether system or non-system.

Closes #17564

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>

lib/libc/newlib/CMakeLists.txt

@@ -3,18 +3,23 @@
 zephyr_library()
 zephyr_library_sources(libc-hooks.c)
 
+# Zephyr normally uses -ffreestanding, which with current GNU toolchains
+# means that the flag macros used by newlib 3.x <inttypes.h> to signal
+# support for PRI.64 macros are not present.  To make them available we
+# need to hook into the include path before the system files and
+# explicitly include the newlib header that provides those macros.
+zephyr_include_directories(include)
+
 # LIBC_*_DIR may or may not have been set by the toolchain. E.g. when
 # using ZEPHYR_TOOLCHAIN_VARIANT=cross-compile it will be either up to the
 # toolchain to know where it's libc implementation is, or if it is
 # unable to, it will be up to the user to specify LIBC_*_DIR vars to
-# point to a newlib implementation.
+# point to a newlib implementation.  Note that this does not change the
+# directory order if LIBC_INCLUDE_DIR is already a system header
+# directory.
 
-# We need to make sure this is included before the standard system
-# header include path's since we build with -ffreestanding and need
-# our libc headers to be picked instead of the toolchain's ffreestanding
-# headers.
 if(LIBC_INCLUDE_DIR)
-  zephyr_system_include_directories(${LIBC_INCLUDE_DIR})
+  zephyr_include_directories(${LIBC_INCLUDE_DIR})
 endif()
 
 if(LIBC_LIBRARY_DIR)

lib/libc/newlib/include/stdint.h

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2019 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#ifndef ZEPHYR_LIB_LIBC_NEWLIB_INCLUDE_STDINT_H_
+#define ZEPHYR_LIB_LIBC_NEWLIB_INCLUDE_STDINT_H_
+
+/* Work around -ffreestanding absence of defines required to support
+ * PRI.64 macros in <inttypes.h> by including the newlib header that
+ * provides the flag macros.
+ */
+
+#include <newlib.h>
+
+#ifdef __NEWLIB__
+/* Has this header.  Older versions do it in <stdint.h>. */
+#include <sys/_stdint.h>
+#endif /* __NEWLIB__ */
+
+/* This should work on GCC and clang.
+ *
+ * If we need to support a toolchain without #include_next the CMake
+ * infrastructure should be used to identify it and provide an
+ * alternative solution.
+ */
+#include_next <stdint.h>
+
+#endif /* ZEPHYR_LIB_LIBC_NEWLIB_INCLUDE_STDINT_H_ */