Plugins: permit building on Windows ARM64

Rather than relying on `sizeof(void *)` to determine the architecture,
use the `CMAKE_SYSTEM_PROCESSOR` variable.  This should allow us to
build for Windows and cross-compile.  Without this, we would attempt to
build the x64 plugin on ARM64 which would fail due to the `CONTEXT` type
being defined for ARM64 rather than `x64`.

llvm-svn: 365155