[ELF][MIPS] Turn Config->Threads off for MIPS targets

For now MipsGotSection class is not ready for concurrent access from
multiple threads. The problem is in the getPageEntryOffset method. It
changes state of MipsGotSection object and might be called from
different threads at the same time. So turn Threads off for this target.

It's a temporary solution. The patch fixes MipsGotSection::getPageEntryOffset
is almost ready.

Differential revision: https://reviews.llvm.org/D27035

llvm-svn: 287740