Exclude routines from fortification

Since the _FORTIFY_SOURCE feature uses some routines of Glibc, they need to
be excluded from the fortification.

On top of that:
 - some tests explicitly verify that some level of fortification works
   appropriately, we therefore shouldn't modify the level set for them.
 - some objects need to be build with optimization disabled, which
   prevents _FORTIFY_SOURCE to be used for them.

Assembler files that implement architecture specific versions of the
fortified routines were not excluded from _FORTIFY_SOURCE as there is no
C header included that would impact their behavior.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>