Support matching signext attribute in the parallel_num_threads_codegen test to...

Support matching signext attribute in the parallel_num_threads_codegen test to appease clang-cmake-mips builder.

The Mips target adds the signext attribute to signed 32-bit integers in order
to support the N32/N64 correctly. Integers must be promoted to 64-bit bit on
these ABI's.

llvm-svn: 222618