DRTVWR-418: Until we figure out how to say FIXED:NO to linker, don't.

The present CMake logic wants to pass FIXED:NO to the linker for 64-bit
builds, which on the face of it seems like a Good Thing: it permits code to be
relocated in memory, preventing collisions if two libraries happen to want to
load into overlapping address ranges.

However the way it's being specified is wrong and harmful. Passing /FIXED:NO
to the compiler command line engages /FI (Forced Include!) of a nonexistent
file XED:NO -- producing lots of baffling fatal compile errors.

Thanks Callum for diagnosing this!