ARM: vdso: Don't use gcc plugins for building vgettimeofday.c

Don't use gcc plugins for building arch/arm/vdso/vgettimeofday.c to
avoid unneeded instrumentation. As previously discussed[1]:

arm_ssp_per_task_plugin.c
	32-bit ARM only (but likely needs disabling for 32-bit ARM vDSO?)

cyc_complexity_plugin.c
	compile-time reporting only

latent_entropy_plugin.c
	this shouldn't get triggered for the vDSO (no __latent_entropy
	nor __init attributes in vDSO), but perhaps explicitly disabling
	it would be a sensible thing to do, just for robustness?

randomize_layout_plugin.c
	this shouldn't get triggered (again, lacking attributes), but
	should likely be disabled too.

sancov_plugin.c
	This should be tracking the KCOV directly (see
	scripts/Makefile.kcov), which is already disabled here.

structleak_plugin.c
	This should be fine in the vDSO, but there's no security
	boundary here, so it wouldn't be important to KEEP it enabled.

[1] https://lore.kernel.org/lkml/20200610073046.GA15939@willie-the-truck/

Signed-off-by: Alexander Popov <alex.popov@linux.com>
Link: https://lore.kernel.org/r/20200624123330.83226-3-alex.popov@linux.com
Signed-off-by: Kees Cook <keescook@chromium.org>

arch/arm/vdso/Makefile

@@ -29,7 +29,7 @@ CPPFLAGS_vdso.lds += -P -C -U$(ARCH)
 CFLAGS_REMOVE_vdso.o = -pg
 
 # Force -O2 to avoid libgcc dependencies
-CFLAGS_REMOVE_vgettimeofday.o = -pg -Os
+CFLAGS_REMOVE_vgettimeofday.o = -pg -Os $(GCC_PLUGINS_CFLAGS)
 ifeq ($(c-gettimeofday-y),)
 CFLAGS_vgettimeofday.o = -O2
 else