Kbuild: compile kernel with gnu11 std

This patch is suggested by linus[1], there may be some corner cases which
should be considered before merge, and is just for prove PATCH 2.

[1]: https://lore.kernel.org/all/CAHk-=wh97QY9fEQUK6zMVQwaQ_JWDvR=R+TxQ_0OYrMHQ+egvQ@mail.gmail.com/

Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>

Makefile

@@ -515,7 +515,7 @@ KBUILD_CFLAGS   := -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs \
 		   -fno-strict-aliasing -fno-common -fshort-wchar -fno-PIE \
 		   -Werror=implicit-function-declaration -Werror=implicit-int \
 		   -Werror=return-type -Wno-format-security \
-		   -std=gnu89
+		   -std=gnu11 -Wno-shift-negative-value
 KBUILD_CPPFLAGS := -D__KERNEL__
 KBUILD_AFLAGS_KERNEL :=
 KBUILD_CFLAGS_KERNEL :=