Fix default ar flags for mingw Windows builds.

Mingw has issues with duplicate entries created when using the default
"qc" flags to the ar command (in cmake). Switching to "crsPD" allows
mingw to generate proper archives that don't rely on extra logic in
binutils to later discard the duplicates.

Bug: http://b/64037266
Test: toolchain/llvm/android/build.py
Change-Id: I5fda433549ab3945220a178ef84211e773a82c7c