lib/gnat: Drop Restriction `No_Exception_Propagation`

It turns out that even with the `-gnatp` switch to suppress runtime
checks, the compiler is still allowed to generate them (it only doesn't
have to). If we can't control generation of checks, we also can't
make assumptions about propagation of their exceptions.

The compiler warning that led to this change seems spurious, though
(the check might be generated, but is dropped later). So we might
revert this decision if the compiler can be fixed.

Change-Id: I7470d74b1f96f90d0d15b24dfd636d5f1c778d46
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/26350
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/lib/gnat/system.ads

@@ -34,7 +34,6 @@
 --                                                                          --
 ------------------------------------------------------------------------------
 
-pragma Restrictions (No_Exception_Propagation);
 pragma Restrictions (No_Exception_Registration);
 
 pragma Restrictions (No_Implicit_Dynamic_Code);