Writing a (valid) C program without main()

Note that this is not a “valid” C program according to the standard. The standard allows for freestanding and hosted environments. Freestanding allows for the entry point to be called something else than main, but there are other requirements. See section 4 and 5 of a draft if interested.

I think the definition of "valid" may differ. Is this a standard conforming C program? No. Did you use implementation details of your platform to create a program that loads and runs correctly? Sure.

Most interesting C programs violate the standard in some way though.

This isn't making a C program. It's using C tools and assembly tools to make an executable.