What do you mean by "AT&T syntax"? Neither 68000 nor SPARC use it, because it's an x86 thing. Do you really mean "operand order"? There's more to an assembly language syntax than just the order of the operands.

AT&T syntax is mainly about operand order, but there are assembler directives and constructs specific to AT&T as found in as(1) on any traditional UNIX, including illumos based ones. That every assembler has his own syntax is nothing new, compare and contrast Master SEKA with ASM-One on Amiga, or MASM, TASM or nasm, for example. AT&T as(1) syntax has nothing to do with x86. Any code written for SVR4 as(1) will be using AT&T syntax, irrespective of processor architecture. ISA used with AT&T as(1) will still be that of the processor, of course, but instead of things like a0 or d0, it'll be %a0 and %d0, for example.