[libc][newlib]在不开启libc的情况下，依然定义RT_USING_NEWLIB和数学库

components/libc/compilers/gcc/newlib/SConscript

@@ -4,18 +4,17 @@ Import('rtconfig')
 src = []
 cwd = GetCurrentDir()
 group = []
-LIBS = []
-CPPDEFINES = []
+LIBS = ['m']
+CPPDEFINES = ['RT_USING_NEWLIB']
 CPPPATH = [cwd]
 
 if rtconfig.PLATFORM == 'gcc':
     if GetDepend('RT_USING_LIBC'):
-        CPPDEFINES += ['RT_USING_NEWLIB']
         # link with libc and libm:
         # libm is a frequently used lib. Newlib is compiled with -ffunction-sections in
         # recent GCC tool chains. The linker would just link in the functions that have
         # been referenced. So setting this won't result in bigger text size.
-        LIBS += ['c', 'm']
+        LIBS += ['c']
 
         src += Glob('*.c')
         if GetDepend('RT_USING_MODULE') == False: