[AVR] Add command line options to selectively remove portions of the RTL startup code.

When writing certain applications (for example bootloaders), compact size is important. In some cases one may not need interrupts in the bootloader, in which case the inclusion of the ISR vector table consume space without adding functionality. In general, removing startup functionality that is definitely not needed can lead to a noticeable size reduction.

This request adds command line options to selectively remove some parts, or even all, the RTL startup code. Using this feature requires a user to fully understand the low level details of the RTL startup code and how the design of the application will be impacted by removal of certain startup code.