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 linkmap map 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.