Solution: use the Win32 compiler to compile for x86_64-linux.
Does it mean that I can simply install 32-bit Lazarus on Win64 and will still be able to compile and debug Win64 apps?
You need to install the 32-bit IDE as well as the Win64
addon. Of course you'll still need to create the units for x86_64-linux... And regarding debugging at least GDB should work, but
fpDebug might not in that case, but
martin_fr will have to answer that.
Can I install 32-bit Lazarus on Lin64 and build/debug Lin64 apps?
On Linux you don't need to do that. From a x86_64-linux-based compiler you can reach any other target (as long as you have the correct compiler and RTL). It's
only x86_64-win64 that's the odd one out on x86 (any other non-x86 platform will have the same problem).