README: update verified toolchain list
improve the descriptions about external sources
enable gcc option -mslow-flash-data to minimize the literal load

gcc-4.9 supports a new compiler option, -mslow-flash-data, which
assumes that loading data from flash is slower than fetching
instruction. Therefore literal load is minimized for better
performance.

GNU Tools for ARM Embedded Processors version 4.8-2014q2-update
backports this option from gcc-4.9, and we introduce cc-option to
detect such new options.