libxx:fix build error when enable LIBCXXTOOLCHAIN

Summary:
  P:➜  nuttx git:(4cec713) LC_ALL=C make
CC:  chip/stm32_gpio.c chip/stm32_gpio.c:41:11: note: '#pragma message: CONFIG_STM32_USE_LEGACY_PINMAP will be deprecated migrate board.h see tools/stm32_pinmap_tool.py'
   41 | #  pragma message "CONFIG_STM32_USE_LEGACY_PINMAP will be deprecated migrate board.h see tools/stm32_pinmap_tool.py"
      |           ^~~~~~~
CXX:  libcxxmini/libxx_new.cxx libcxxmini/libxx_new.cxx:72:11: error: redefinition of 'void* operator new(std::size_t, void*)'
   72 | FAR void *operator new(std::size_t nbytes, FAR void *ptr)
      |           ^~~~~~~~
In file included from /usr/include/newlib/c++/13.2.1/bits/atomic_base.h:36,
                 from /usr/include/newlib/c++/13.2.1/atomic:41,
                 from /home/leduc/wdc_workspace/src/autre/nuttx_dev/nuttx/include/nuttx/atomic.h:32,
                 from /home/leduc/wdc_workspace/src/autre/nuttx_dev/nuttx/include/nuttx/fs/fs.h:38,
                 from /home/leduc/wdc_workspace/src/autre/nuttx_dev/nuttx/include/nuttx/lib/lib.h:30,
                 from libcxxmini/libxx_new.cxx:29:
/usr/include/newlib/c++/13.2.1/new:174:33: note: 'void* operator new(std::size_t, void*)' previously defined here
  174 | _GLIBCXX_NODISCARD inline void* operator new(std::size_t, void* __p) _GLIBCXX_USE_NOEXCEPT
      |                                 ^~~~~~~~
make[1]: *** [Makefile:69: libxx_new.o] Error 1
make: *** [tools/LibTargets.mk:216: libs/libxx/libxx.a] Error 2

Signed-off-by: chenrun1 <[email protected]>

libs/libxx/Makefile

@@ -33,12 +33,13 @@ ifeq ($(CONFIG_UCLIBCXX),y)
 include uClibc++.defs
 else ifeq ($(CONFIG_LIBCXX),y)
 include libcxx.defs
-else
+else ifeq ($(CONFIG_LIBCXXMINI),y)
 include libcxxmini.defs
+endif
+
 ifeq ($(CONFIG_ETL),y)
 include etl.defs
 endif
-endif
 
 ifeq ($(CONFIG_LIBCXXABI),y)
 include libcxxabi.defs