Linux kernel minimum compiler raised to GCC 5.1, allowing potential C11 use

Change also has handy side effect of fixing some warnings as errors

Linux creator and maintainer Linus Torvalds has merged a late change to the forthcoming 5.15 kernel code that raises the minimum compiler from GCC 4.9 to 5.1 - which may in future enable use of an updated version of the C programming language, C11....