[otbn] Ensure the RIG always picks aligned jumps

The choice of jump target is made in Program.pick_branch_targets,
which was mostly careful to always pick aligned addresses. However, it
didn't correctly handle its tgt_min/tgt_max arguments, which might not
be aligned.

Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>

hw/ip/otbn/util/rig/gens/jump.py

diff --git a/hw/ip/otbn/util/rig/gens/jump.py b/hw/ip/otbn/util/rig/gens/jump.py
index a8eccec..b8fbbda 100644
--- a/hw/ip/otbn/util/rig/gens/jump.py
+++ b/hw/ip/otbn/util/rig/gens/jump.py
@@ -109,6 +109,7 @@
         if tgt is None:
             return None
         assert tgt_min <= tgt <= tgt_max
+        assert tgt & 3 == 0
 
         # Adjust again for base_addr: we pick the offset from there
         op_val = tgt - base_addr

hw/ip/otbn/util/rig/program.py

diff --git a/hw/ip/otbn/util/rig/program.py b/hw/ip/otbn/util/rig/program.py
index fae42b4..c3f1876 100644
--- a/hw/ip/otbn/util/rig/program.py
+++ b/hw/ip/otbn/util/rig/program.py
@@ -346,6 +346,13 @@
 
         '''
 
+        # Round tgt_min and tgt_max inwards to multiples of 4 (because
+        # instructions always have to be word aligned)
+        if tgt_min is not None:
+            tgt_min = (tgt_min + 3) & ~3
+        if tgt_max is not None:
+            tgt_max = tgt_max & ~3
+
         # To pick the targets, we start by choosing a "gap" between existing
         # sections in which they should land. To do *that*, we start by making
         # a list of all the gaps between sections in ascending order of base