[LTO] Use llvm-nm instead of nm in new tests

The use of nm in the new tests added with r281725 caused a couple
of bot failures:
http://lab.llvm.org:8011/builders/clang-x64-ninja-win7/builds/15701
http://bb.pgr.jp/builders/ninja-clang-i686-msc19-R/builds/6939

Use llvm-nm instead.

llvm-svn: 281750