It's safe almost always, unless A) you're doing something really undefined-behavior-ish and strange... or, here, for iPXE, I'm guessing the problem is B) you're running code that it emits assuming it's running on the Linux kernel in a fully initialized environment in a preboot/embedded environment.. and still expect setjmp/longjmp to work.

That is, it doesn't make sense with -ffreestanding, but Ubuntu enables it in that case.