link with libm.so

This fix the compiling error:

    /usr/lib64/gcc/x86_64-slackware-linux/4.7.2/../../../../x86_64-slackware-linux/bin/ld: build/components/rtgui/common/dc.o: undefined reference to symbol 'sin@@GLIBC_2.2.5'
    /usr/lib64/gcc/x86_64-slackware-linux/4.7.2/../../../../x86_64-slackware-linux/bin/ld: note: 'sin@@GLIBC_2.2.5' is defined in DSO /lib64/libm.so.6 so try adding it to the linker command line
    /lib64/libm.so.6: could not read symbols: Invalid operation
    collect2: error: ld returned 1 exit status

bsp/simulator/SConstruct

@@ -55,6 +55,7 @@ else:
     TARGET = 'rtthread'
     env.Append(CCFLAGS=rtconfig.CFLAGS)
     env.Append(LINKFLAGS=rtconfig.LFLAGS)
+    env.Append(LIBS=['m'])
 
 
 # prepare building environment