Explicitly add -target for Windows builds in file_test_windows.c

It turns out that the llvm buildbots run the test with
-DLLVM_DEFAULT_TARGET_TRIPLE=x86_64-scei-ps4, which would cause this
test to fail as the test assumed that the default target is Windows. To
fix this, we explicitly set -target for the Windows testcases.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D125425