WIP/RFC: Add support for the Texas Instruments Tiva TM4C123GLX ARM Cortex-M4F MCU

This adds basic support for the TI TM4C123GH6PM MCU and the TM4C123GLX Launchpad development board based on it.

[TM4C123GH6PM](http://www.ti.com/product/tm4c123gh6pm) is a Cortex-M4F based chip with 32kiB Ram, running at up to 80 MHz.
As a special feature it provides 'ROM Functions', similar to BIOS calls, that are always present on the chip and that can be called using a memory table.

There are 6 32 bit general purpose timers and 6 64 bit general purpose timers.
Timers can be 'split' into two half-with timers (16/32bit). Prescaler is only available in half-with mode when counting down, when counting up it will extend the range by 8 bit.

I used stm32f4 as a template for this port.

working so far:
 - UART (tx only)
 - timers (32bit timers in full width mode are exported through hwtimer, no prescaler), tests/vtimer_msg crashes after ~53s

Depends on #1709 to run properly
cpu/tm4c123/syscalls.c:92: style (unsignedLessThanZero): Checking if unsigned variable 'incr' is less than zero.