Clean up GCC_ARM startup code

This pull request cleans up startup code for GCC_ARM in following ways:
1. Prefixing internal labels with .L to avoid making them external symbols. Making dissemble version more friendly
2. Reuse the single irq default handler for all irq handlers, to avoid code size waste from replicating all irq handlers.
3. For STM32F407, needn't clear BSS region as it will be cleared in startup code of C library (_start)

Tested all board with GCC_ARM with no more build failure. 
Tested with LPC1768.
It doesn't impact any tools other than GCC_ARM.