添加rtthread相关代码

riscv/rtthread/components/libc/posix/io/poll/SConscript

@@ -0,0 +1,17 @@
+# RT-Thread building script for component
+
+from building import *
+
+cwd     = GetCurrentDir()
+src     = []
+CPPPATH = [cwd]
+
+if GetDepend('RT_USING_POSIX_POLL'):
+    src += ['poll.c']
+
+if GetDepend('RT_USING_POSIX_SELECT'):
+    src += ['select.c']
+
+group = DefineGroup('POSIX', src, depend = [''], CPPPATH = CPPPATH)
+
+Return('group')