Fix FORTIFY_SOURCE unittests.

The compiler is too damn smart.

Change-Id: Ibef3ef41ec99f8cd9c06f1dbca535819f9a08197
Reorganize FORTIFY_SOURCE tests.

Get rid of a lot of the duplication in the various FORTIFY_SOURCE
tests. Instead, we build 4 separate static libraries, with
4 different compile time options, and link them into the final test
binary.

Change-Id: Idb0b7cccc8dd837adb037bf4ddfe8942ae138230