Raise the minimum required gcc version to 4.6

Raise the minimum required gcc version to 4.6

Various architectures fail to build properly with older versions of the
gcc compiler.

An example from Guenter Roeck in thread [1]:
>
>   In file included from ./include/linux/mm.h:17:0,
>                    from ./include/linux/pid_namespace.h:7,
>                    from ./include/linux/ptrace.h:10,
>                    from arch/openrisc/kernel/asm-offsets.c:32:
>   ./include/linux/mm_types.h:497:16: error: flexible array member in otherwise empty struct
>
> This is just an example with gcc 4.5.1 for or32. I have seen the problem
> with gcc 4.4 (for unicore32) as well.

So update the minimum required version of gcc to 4.6.

[1] https://lore.kernel.org/lkml/20180814170904[email protected]/

Miscellanea:

 - Update Documentation/process/changes.rst

 - Remove and consolidate version test blocks in compiler-gcc.h for
   versions lower than 4.6

Signed-off-by: Joe Perches <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Reviewed-by: Nick Desaulniers <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>