Try to fix gdb cannot access problem, but still fail.

do the following work:
- don't compile FreeRTOS related file(change in Makefile)
- compile stm32f4xx_it.c(by change in Makefile)
- change HardFault_Handler in stm32f4xx_it.c, add some asm code suggested from http://koti.kapsi.fi/jpa/stuff/other/stm32-hardfault-backtrace.html

but still not work