ignore lto-wrapper warnings for lto builds.

  I see these warnings from time to time, when configuring a build with  --enable-pgo-build=lto, I haven't yet found out why I see these sometime, and  why not. E.g. https://gcc.gnu.org/PR109241. Just ignore these when they appear  in test cases. lto-wrapper: warning: using serial compilation of N LTRANS jobs