llvm-fake.py tweaks: don't use an environment var to specify location of llc, since scons srubs the environment.  Use a symlink instead, and issue an informative error if it is missing.
llvm-fake.py tweaks: don't use an environment var to specify location
of llc, since scons srubs the environment.  Use a symlink instead,
and issue an informative error if it is missing.

Also, improve a couple of comments.

Review URL: http://codereview.chromium.org/1966001

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@2202 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,sbc100/native_client,nacl-webkit/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client