ignore -flto= options recognized by GCC

as requested in https://bugs.llvm.org/show_bug.cgi?id=49553, submitting the proposed changes to just ignore the -flto= options which are recognized by GCC ("auto" and "jobserver").

GCC supports -flto=<auto|jobserver|<N> to select the parallelity for LTO builds. LLVM also has -flto-jobs=<N>, which only seems to have a meaning when used with -flto=thin?

The attached patch just ignores the values "auto" and "jobserver". that doesn't change anything in functionality.  Another option would be to map these values to either "thin" or "full", maybe in presence of the -ffat-lto-objects option?

-flto=<n> could also be translated to -flto-jobs=<N>.

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D99501