Add conditional branch encodings for RISC-V.

Not all br_icmp opcodes are present in the ISA. The missing ones can be
reached by commuting operands.

Don't attempt to encode EBB offsets yet. For now just emit an EBB
relocation for the branch instruction.

cranelift/test-all.sh

@@ -56,7 +56,7 @@ fi
 if [ -n "$needcheck" ]; then
     banner $(python --version 2>&1)
     $topdir/lib/cretonne/meta/check.sh
-    touch $tsfile
+    touch $tsfile || echo no target directory
 fi
 
 PKGS="cretonne cretonne-reader cretonne-tools filecheck"