Switch to using --Wl,--allow-multiple-definition on Windows.

It turns out that we can just suppress the linker warning (plus -Werror)
for multiply defined symbols on windows 64-bit builds. Our version of
mingw is old enough to not recognize many of these functions as
identical from separate sub-components. This change removes existing
unnecessary CMake variables that previously looked like they were
sufficient, but it turned out that multiple calls to ninja were masking
the actual error (since the output files would sometimes still get
created)!

Bug: http://b/62787860
Test: toolchain/llvm/android/build.py
Change-Id: Ie118ba91f83d71c8f51eff9c132d5b1eb5ba8897