Add printf/scanf -Wformat checking

[re-send as outgoing mail setup had a DKIM issue pointed out
by Linus]

This series (which was initially sent back in 2020) adds the
ability to deal with __attribute__((format) and checking the
arguments to formatted variadic functions.

I have been considering adding a -Wformat-linux to this as
the kernel now has a number of extra formatting options and
checking the type of these would be useful. An even nicer
extension would be some way of informing the compiler/sparse
of these at compile time (but would need to get agreement on
how to do this with the compilers too)

Another quesiton is to leave this as 4 patches or merge down?