Fix redundant comparison in gcc::Common::ConstructJob.

We were checking "Arch == llvm::Triple::x86_64 || Arch
== llvm::Triple::x86_64", but the rhs should actually check for
powerpc64.

Found while experimenting with a potential new Clang warning.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177496 91177308-0d34-0410-b5e6-96231b3b80d8