Clear __USE_FORTIFY_LEVEL after push it to stack.

We need to undefine __USE_FORTIFY_LEVEL when including 'string.h'
after storing the old value to stack. Previously I forget to clean
the value thus __USE_FORTIFY_LEVEL still takes effect.

Tests: Add -O2 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -g to
GENERIC_CPPFLAGS in trusty/kernel/make/host_test.mk, make sure
the gcc build passes without error.
Bug: None
Change-Id: Ifa0b7ef20d68ac73113ab787e125631105bf449c