Linux kernel switches from C89 to C11 (or rather, -std=gnu11)

The impetus for the switch stems in part from an issue in the helper macro list_for_each_entry() -- an issue that could be alleviated if declaring variables in a 'for' loop header was allowed [1].

[1] https://lore.kernel.org/lkml/86C4CE7D-6D93-456B-AA82-F8ADEAC...