So System V came out in 1983, which postdates the introduction of the x86 architecture because the IBM PC was x86 from day one and it first came out in 1981. (I'm sure the 8086 is older, but it doesn't really matter here.)
Of course, it isn't the only standard out there for x86. Microsoft exists, after all.
Anyway, all of these standards have to specify which registers get saved by the called function and which get clobbered (and, therefore, must be saved by the calling function if their values are important) and you can't know which registers will exist on a given ISA until it exists, so the specific System V ABI for the x86 couldn't possibly have existed until the chips did. Ditto the System V ABI for the x86-64, the MIPS, and all of the other ISAs for which there is such a document.
However, chip designers aren't stupid. They know that their chips will be used to run binaries generated by C compilers, and that C compilers like stack frames and registers. Therefore, modern ISAs are designed with those things in mind, and I wouldn't be surprised if at least a few chip designers worked with one eye on some ABI documentation.
(Compare modern chips with older minicomputer and mainframe ISAs, where call stacks weren't always available and where subroutine calls sometimes involved self-modifying code. The PDP-8, for example, did subroutine calls by writing the the return address to the first (12-bit) word of memory in the subroutine, and then jumping to the address just past it; returning was a simple matter of jumping to a memory location loaded from a known address, but recursion was effectively impossible. The PDP-8 was a fine machine for assembly language programmers, but almost perversely unfriendly to modern mid-level languages, such as C and Pascal.)
Burroughs, IBM mainframes, Ada Machines (by Rational), Xerox Parc and ETHZ workstations are probably the best examples of more higher level friendly architectures.
http://wiki.osdev.org/System_V_ABI
So System V came out in 1983, which postdates the introduction of the x86 architecture because the IBM PC was x86 from day one and it first came out in 1981. (I'm sure the 8086 is older, but it doesn't really matter here.)
Of course, it isn't the only standard out there for x86. Microsoft exists, after all.
https://en.wikipedia.org/wiki/X86_calling_conventions
Anyway, all of these standards have to specify which registers get saved by the called function and which get clobbered (and, therefore, must be saved by the calling function if their values are important) and you can't know which registers will exist on a given ISA until it exists, so the specific System V ABI for the x86 couldn't possibly have existed until the chips did. Ditto the System V ABI for the x86-64, the MIPS, and all of the other ISAs for which there is such a document.
However, chip designers aren't stupid. They know that their chips will be used to run binaries generated by C compilers, and that C compilers like stack frames and registers. Therefore, modern ISAs are designed with those things in mind, and I wouldn't be surprised if at least a few chip designers worked with one eye on some ABI documentation.
(Compare modern chips with older minicomputer and mainframe ISAs, where call stacks weren't always available and where subroutine calls sometimes involved self-modifying code. The PDP-8, for example, did subroutine calls by writing the the return address to the first (12-bit) word of memory in the subroutine, and then jumping to the address just past it; returning was a simple matter of jumping to a memory location loaded from a known address, but recursion was effectively impossible. The PDP-8 was a fine machine for assembly language programmers, but almost perversely unfriendly to modern mid-level languages, such as C and Pascal.)