This builds for windows on arm, with llvm-mingw. The target triplet
is named -gcc since that's how similar existing targets are named,
even though it technically runs clang (via frontends named
"$CROSS-gcc").
Assemble using $CC -c since there's no standalone assembler
available (except perhaps llvm-mc).
Change-Id: I2c9a319730afef73f811bad79f488dcdc244ab0d