enable LTO : add (used) attribute to IT handler

See http://www.coocox.org/forum/topic.php?id=3002 for an explanation
enable lto in Release mode

tested with led example, for stm32f4 and stm32f100 (the example does not
work on stm32f103).
On stm32f100 :
text size shrinks from 2876 to 2628
dec  size shrinks from 8076 to 7824
data size shrinks from 40 to 36
bss unchanged