arch: posix: Undefine operating system specific macros for native_sim

Compilers predefine system-specific macros which carry information about
compiler, target architecture and operating system. It provides basic
compiler-dependent information like size of types, their maximal and
minimal values, etc. It allows to write common libc headers for multiple
architectures and operating systems.

These macros allow code to always determine what is the target operating
system. This is a problem when compiling code of modules that supports
multiple operating systems (e.g. cryptography libraries).

To avoid confusion we shouldn't leak host operating system macros (e.g.
__linux__, __linux, linux, etc.) when compiling for native_sim board.

Unfortunately, there is no single universal switch that disables all
operating system macros:
- '-undef' removes also architecture-related macros
- '--target' is only available for Clang compiler

This patch uses '-include' option to include file that undefines all
well-known operating system macros.

Run 'gcc -dM -E - < /dev/null | sort' to get full list of predefined
macros.

Signed-off-by: Patryk Duda <patrykd@google.com>

arch/posix/CMakeLists.txt

@@ -99,11 +99,13 @@ elseif (CONFIG_NATIVE_LIBRARY)
     get_filename_component(COMPILER_OWN_INCLUDE_PATH "${_OUTPUT}" DIRECTORY)
 
     # Do not use the C library from this compiler/host,
-    # but still use the basic compiler headers
+    # but still use the basic compiler headers,
+    # remove all operating system specific predefined macros,
     # no_builtin to avoid the compiler using builtin replacements for std library functions
     zephyr_compile_options(
       -nostdinc
       -isystem ${COMPILER_OWN_INCLUDE_PATH}
+      "SHELL:-include ${ZEPHYR_BASE}/arch/posix/include/undef_system_defines.h"
       $<TARGET_PROPERTY:compiler,freestanding>
       $<TARGET_PROPERTY:compiler,no_builtin>
     )

arch/posix/include/undef_system_defines.h

@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2024 Google LLC
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/*
+ * Undefine all system-specific macros defined internally, by the compiler.
+ * Run 'gcc -dM -E - < /dev/null | sort' to get full list of internally
+ * defined macros.
+ */
+
+#undef __gnu_linux__
+#undef __linux
+#undef __linux__
+#undef linux
+
+#undef __unix
+#undef __unix__
+#undef unix