clang/test/Modules/self-import-header/test.m: Add explicit -target.

Clang has an issue between mingw/include/float.h and clang/Headers/float.h with cyclic include_next.
For now, it should work to suppress #include_next in clang/float.h with an explicit target.
(It may work with -U__MINGW32__, though.)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181988 91177308-0d34-0410-b5e6-96231b3b80d8