Subzero: Fix dependency checking to avoid unnecessary rebuilds.

When trying to do bisection debugging, the pnacl-llc translation was happening every time even if the pexe didn't change.  This is because it was checking for a binary called 'llc' in the current directory, instead of an absolute path the pnacl-llc.  (This check is done so that updating pnacl-llc triggers a rebuild of the bisection binary, similar to the check for an update of pnacl-sz.)

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/1044623003
diff --git a/pydir/szbuild.py b/pydir/szbuild.py
index a067232..d717c5f 100755
--- a/pydir/szbuild.py
+++ b/pydir/szbuild.py
@@ -172,7 +172,7 @@
     pnacl_sz = (
         '{root}/toolchain_build/src/subzero/pnacl-sz'
         ).format(root=nacl_root)
-    llcbin = 'llc'
+    llcbin = '{base}/pnacl-llc'.format(base=path_addition)
     gold = 'le32-nacl-ld.gold'
     opt_level = args.optlevel
     opt_level_map = { 'm1':'0', '-1':'0', '0':'0', '1':'1', '2':'2' }