lib: cpp: Relocate C++ ABI library components to lib/cpp/abi

This commit relocates the "C++ ABI library" components such as
global constructor/destructor and initialiser handlers to a dedicated
directory, `lib/cpp/abi`, in order to provide a clear separation
between the C++ ABI/runtime library and the standard C++ library
components.

Note that the Zephyr C++ ABI library currently implements the GNU/GCC
C++ ABI, which is the de-facto standard ABI used by many compilers
including Clang -- it may be necessary to sub-divide the `lib/cpp/abi`
into `lib/cpp/abi/gnu` and `lib/cpp/abi/someotherabi` in the future
when adding the support for a C++ compiler that expects an ABI vastly
different from the GNU C++ ABI.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>

lib/cpp/CMakeLists.txt

@@ -1,12 +1,6 @@
 # SPDX-License-Identifier: Apache-2.0
 
-zephyr_sources(cpp_init.c)
-
-zephyr_sources_ifdef(CONFIG_CPP_STATIC_INIT_GNU
-  cpp_init_array.c
-  cpp_ctors.c
-  cpp_dtors.c
-)
+add_subdirectory(abi)
 
 if (NOT CONFIG_LIB_CPLUSPLUS)
 zephyr_system_include_directories(

lib/cpp/abi/CMakeLists.txt

@@ -0,0 +1,9 @@
+# SPDX-License-Identifier: Apache-2.0
+
+zephyr_sources(cpp_init.c)
+
+zephyr_sources_ifdef(CONFIG_CPP_STATIC_INIT_GNU
+  cpp_init_array.c
+  cpp_ctors.c
+  cpp_dtors.c
+)