Check that symbol names in symbol tables are unique.

Makes sure that names within a symbol table are unique. Also cleans
up error reporting to be more consistent between symbol tables.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4301
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1347683003 .
3 files changed