x86: Fix disassembly of syscall instructions.

These *used* to have a source register which was the syscall number.
That was not an actual property of the instruction, it was a property of
the syscall mechanism in Linux which had leaked into the implementation
of the instructions themselves.

That was corrected by removing that source register and letting the
syscall handling layer figure out which system call to invoke.

Unfortunately the SyscallInst format wasn't also updated, and when
printing the disassembly for those instructions, it would try to print
that source register. That instruction doesn't bother to set up a source
operand array since it doesn't actually have any, so that ended up
accessing random memory.

Change-Id: I20890ad69605983305a13f414ca59a5fa35d4d06
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49183
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/x86/isa/formats/syscall.isa

@@ -64,10 +64,7 @@ output decoder {{
             Addr PC, const loader::SymbolTable *symtab) const
     {
         std::stringstream response;
-
         printMnemonic(response, mnemonic);
-        ccprintf(response, " ");
-        printReg(response, srcRegIdx(0), machInst.opSize);
         return response.str();
     }
 }};