Adding target list to --help.
Adding target list to --help.

Adding a list of available packages to the --help message.

BUG=http://code.google.com/p/nativeclient/issues/detail?id=3086
TEST=manual
R=ncbray@google.com

Review URL: https://codereview.chromium.org/11361039

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@10201 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

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