add minimal LM4F120H5QR (LM4F120XL Stellaris Launchpad) support

include/arch/lm4f120h5qr-stellaris/driver/gpio.h

include/arch/lm4f120h5qr-stellaris/driver/stdout.h

include/arch/lm4f120h5qr-stellaris/driver/uptime.h

include/arch/lm4f120h5qr-stellaris/lm4f_cpp_wrapper.h

src/arch/lm4f120h5qr-stellaris/Makefile.inc

src/arch/lm4f120h5qr-stellaris/arch.cc

src/arch/lm4f120h5qr-stellaris/driver/gpio.cc

src/arch/lm4f120h5qr-stellaris/driver/stdout.cc

src/arch/lm4f120h5qr-stellaris/driver/uptime.cc

src/arch/lm4f120h5qr-stellaris/lm4f120h5qr.ld

src/arch/lm4f120h5qr-stellaris/lm4f_cpp_wrapper.c

src/arch/lm4f120h5qr-stellaris/prompt