Add generic Cortex-M startup code and linker script.

Based on http://dev.frozeneskimo.com/notes/cortex_cmsis/
Add NO_SYSTEMINIT define to skip call to SystemInit().
Correction: any Cortex-M CPU.
Add few MCU-specific memory map link files.
Add license file with some hints.
Add an example on how to build binaries using cortex-startup.
Add linkscript for Nordic nRF51.
Add header file with IRQ handlers declarations, useful for C++.
Add linkscript for lm4f120h5qr (Stellaris Launchpad).
Add Energy Micro EFM32 Giant Gecko IRQ list.
Add simple util to convert IRQ list into startup C file.
Add Energy Micro EFM32 Tiny Gecko IRQ list.
Add Energy Micro EFM32 Gecko IRQ list.
Add Energy Micro EFM32 Leopard Gecko IRQ list.
Add Energy Micro EFM32 Wonder Gecko IRQ list.
Support IRQ list with explicit IRQ numbers.

Useful for MCUs with gaps in IRQ vectors. Don't switch all IRQ list to new
format, because it's usually easier to diff lists without explicit numbers.
(It seems there're 2 vendor approaches here: 1) don't use gaps, just renumber
supported vectors (EFM32), 2) use same numbering for IRQs, just use gaps if
specific model doesn't support some IRQs (probably most other vendors)).
Add STM32F100 IRQ list.
irqgen.py: Write output to startup-*.c file.
Add script to regenerate all startup-*.c files from IRQ lists.
Move link script into MCU family dirs.