add tm4c123 SoC support

Add TI's Cortex M4 'tiva'
add ek-tm4c123gxl launchpad based on tm4c123
riot_and_cpp: add ek-tm4c123gxl to blacklist

since every board but native is on the blacklist…
reduce the stack size so test/posix_semaphore builds

compared to other implementations, 2500 seems to be insanely high anyway
introduce 'software prescaler' with overflow counter

The 32bit timer overflows every 53s, shift it down and count the interrupts for a greater range