cmake(bugfix):add missing c++ toolchain search path

when libcxx or uClib++ is not selected, we use the default NUTTX cxx

Signed-off-by: xuxin19 <xuxin19@xiaomi.com>

arch/arm/src/cmake/gcc.cmake

@@ -247,10 +247,8 @@ if(CONFIG_BUILD_PIC)
   add_link_options(-Wl,--emit-relocs)
 endif()
 
-add_compile_options(
-  -Wno-attributes -Wno-unknown-pragmas
-  $<$<COMPILE_LANGUAGE:C>:-Wstrict-prototypes>
-  $<$<COMPILE_LANGUAGE:CXX>:-nostdinc++>)
+add_compile_options(-Wno-attributes -Wno-unknown-pragmas
+                    $<$<COMPILE_LANGUAGE:C>:-Wstrict-prototypes>)
 
 # When all C++ code is built using GCC 7.1 or a higher version, we can safely
 # disregard warnings of the type "parameter passing for X changed in GCC 7.1."
@@ -259,6 +257,10 @@ add_compile_options(
 
 add_compile_options(-Wno-psabi)
 
+if(NOT CONFIG_LIBCXXTOOLCHAIN)
+  add_compile_options($<$<COMPILE_LANGUAGE:CXX>:-nostdinc++>)
+endif()
+
 if(CONFIG_CXX_STANDARD)
   add_compile_options($<$<COMPILE_LANGUAGE:CXX>:-std=${CONFIG_CXX_STANDARD}>)
 endif()