There's two options to link the compiled object files that ldc spits out: with avr-gcc's linker, or with llvm's internal linker. The argument -gcc=avr-gcc tells it where to find avr-gcc tools, or you can use --link-internally instead, which uses llvm's built-in linker. Both will work.
The nice thing about using the -gcc thing is you can point it at your arduino studio install - which you probably want anyway since it has the objcopy, avrdude, etc. programs as well as other useful things - and have it just work, and integrate better with other libraries and tools and such, like the C runtime that gcc provides.
$ PATH=/opt/arduino/arduino-1.8.10/hardware/tools/avr/bin:$PATH ldc2 -betterC -Oz -mtriple=avr -mcpu=atmega328p -gcc=avr-gcc delay.d
$ file delay
delay: ELF 32-bit LSB executable, Atmel AVR 8-bit, version 1 (SYSV), statically linked, with debug_info, not stripped
$ ls -l delay
-rwxr-xr-x 1 me users 2440 Sep 18 08:28 delay
With the arduino tools in the PATH, the -gcc=avr-gcc argument just works.
With --link-internally, you don't need any of that, but:
$ ldc2 -betterC -Oz -mtriple=avr -mcpu=atmega328p --link-internally delay.d
lld: warning: cannot find entry symbol _start; defaulting to 0x110B4
$ file delay
delay: ELF 32-bit LSB executable, Atmel AVR 8-bit, version 1 (SYSV), statically linked, not stripped
$ ls -l delay
-rwxr-xr-x 1 me users 708 Sep 18 08:31 delay
Notice it linked, but it gave a warning about the missing _start. Also notice the different file size. This is because the C runtime stubs that gcc provides has that start symbol, which does some basic setup then calls your extern(C) main. Without that, you will need to do a little more yourself. I'm sure you can get ldc to link in that too by listing the lib/object files in the link command... but with -avr-gcc, it just works since it already knows where to find all that stuff.
