[libc] 调整文件结构

components/libc/compilers/common/extension/readme.md

+## Attentions
+
+This folder is "common" for toolchains, which only support ISO C, as an extension part, such as Keil-MDK and IAR.
+

components/libc/compilers/common/nogcc/readme.md

-## Attentions
-
-This folder is "common" for toolchains excluding gcc.
\ No newline at end of file

components/libc/compilers/gcc/newlib/legacy/SConscript

-# RT-Thread building script for bridge
-
-import os
-from building import *
-
-cwd = GetCurrentDir()
-objs = []
-list = os.listdir(cwd)
-
-for d in list:
-    path = os.path.join(cwd, d)
-    if os.path.isfile(os.path.join(path, 'SConscript')):
-        objs = objs + SConscript(os.path.join(d, 'SConscript'))
-
-Return('objs')