Kbuild: move to -std=gnu11

During a patch discussion, Linus brought up the option of changing
the C standard version from gnu89 to gnu99, which allows using variable
declaration inside of a for() loop. While the C99, C11 and later standards
introduce many other features, most of these are already available in
gnu89 as GNU extensions as well.

An earlier attempt to do this when gcc-5 started defaulting to
-std=gnu11 failed because at the time that caused warnings about
designated initializers with older compilers. Now that gcc-5.1 is
the minimum compiler version used for building kernels, that is no
longer a concern. Similarly, the behavior of 'inline' functions changes
between gnu89 using gnu_inline behavior and gnu11 using standard c99+
behavior, but this was taken care of by defining 'inline' to include
__attribute__((gnu_inline)) in order to allow building with clang a
while ago.

Nathan Chancellor reported a new -Wdeclaration-after-statement
warning that appears in a system header on arm, this still needs a
workaround.

The differences between gnu99, gnu11, gnu1x and gnu17 are fairly
minimal and mainly impact warnings at the -Wpedantic level that the
kernel never enables. Between these, gnu11 is the newest version
that is supported by all supported compiler versions, though it is
only the default on gcc-5, while all other supported versions of
gcc or clang default to gnu1x/gnu17.

Link: https://lore.kernel.org/lkml/CAHk-=wiyCH7xeHcmiFJ-YgXUy2Jaj7pnkdKpcovt8fYbVFW3TA@mail.gmail.com/
Link: https://github.com/ClangBuiltLinux/linux/issues/1603
Suggested-by: Linus Torvalds <[email protected]>
Acked-by: Marco Elver <[email protected]>
Acked-by: Jani Nikula <[email protected]>
Acked-by: David Sterba <[email protected]>
Tested-by: Sedat Dilek <[email protected]>
Reviewed-by: Alex Shi <[email protected]>
Reviewed-by: Nick Desaulniers <[email protected]>
Reviewed-by: Miguel Ojeda <[email protected]>
Signed-off-by: Arnd Bergmann <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Signed-off-by: Masahiro Yamada <[email protected]>

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