build: repair cross-compilation with clang

When building for Windows, we would check if we were using MSVC rather
than WIN32.  This resulted in needed targets not being defined by
sanitizer_common.  Fix the conditional.

When registering the objects libraries for ASAN, we would multiply
register for all targets as we were creating them inside a loop over all
architectures.  Only define the target per architecture.

llvm-svn: 294510