ARM: multi_v7_defconfig: enable GCC plugin support and structleak plugin

ARM: multi_v7_defconfig: enable GCC plugin support and structleak plugin

Enable the base GCC plugin support symbol CONFIG_GCC_PLUGINS for
multi_v7_defconfig so that builds hosted on systems that have a
supported version of GCC along with the prerequisite headers will
allow GCC plugins to be used during the build. At the moment, this
will also enable the ARM per-task stack protector plugin, since
all its remaining Kconfig dependencies are fulfilled by default.

At the same time, enable the structleak plugin. This plugin ensures
that local struct type variables are fully initialized, preventing
inadvertent leaks of kernel state if such variables are shared with
userland. This is a reasonable transformation to apply by default,
since it does not adversely affect performance, and does not affect
GCC compiler diagnostics regarding the use of uninitialized local
variables.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>