ARM: 8961/2: Fix Kbuild issue caused by per-task stack protector GCC plugin

When using plugins, GCC requires that the -fplugin= options precedes
any of its plugin arguments appearing on the command line as well.
This is usually not a concern, but as it turns out, this requirement
is causing some issues with ARM's per-task stack protector plugin
and Kbuild's implementation of $(cc-option).

When the per-task stack protector plugin is enabled, and we tweak
the implementation of cc-option not to pipe the stderr output of
GCC to /dev/null, the following output is generated when GCC is
executed in the context of cc-option:

  cc1: error: plugin arm_ssp_per_task_plugin should be specified before \
         -fplugin-arg-arm_ssp_per_task_plugin-tso=1 in the command line
  cc1: error: plugin arm_ssp_per_task_plugin should be specified before \
         -fplugin-arg-arm_ssp_per_task_plugin-offset=24 in the command line

These errors will cause any option passed to cc-option to be treated
as unsupported, which is obviously incorrect.

The cause of this issue is the fact that the -fplugin= argument is
added to GCC_PLUGINS_CFLAGS, whereas the arguments above are added
to KBUILD_CFLAGS, and the contents of the former get filtered out of
the latter before being passed to the GCC running the cc-option test,
and so the -fplugin= option does not appear at all on the GCC command
line.

Adding the arguments to GCC_PLUGINS_CFLAGS instead of KBUILD_CFLAGS
would be the correct approach here, if it weren't for the fact that we
are using $(eval) to defer the moment that they are added until after
asm-offsets.h is generated, which is after the point where the contents
of GCC_PLUGINS_CFLAGS are added to KBUILD_CFLAGS. So instead, we have
to add our plugin arguments to both.

For similar reasons, we cannot append DISABLE_ARM_SSP_PER_TASK_PLUGIN
to KBUILD_CFLAGS, as it will be passed to GCC when executing in the
context of cc-option, whereas the other plugin arguments will have
been filtered out, resulting in a similar error and false negative
result as above. So add it to ccflags-y instead.

Fixes: 189af465 ("ARM: smp: add support for per-task stack canaries")
Reported-by: Merlijn Wajer <merlijn@wizzup.org>
Tested-by: Tony Lindgren <tony@atomide.com>
Acked-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>