x86: Define a local ABI for system calls.

These ABIs (one 32 bit and one 64 bit) take advantage of the
GenericSyscallABI and X86Linux::SyscallABI partial ABIs set up earlier.

This removes x86's dependence on the getSyscallArg and setSyscallReturn
Process methods.

Change-Id: Ia07834cea1afa827d77e590af5397e2a1e0e2099
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23443
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/x86/linux/linux.hh

@@ -65,9 +65,7 @@ class X86Linux : public Linux
             ctc->setIntReg(X86ISA::StackPointerReg, stack);
     }
 
-    class SyscallABI
-    {
-    };
+    class SyscallABI {};
 };
 
 namespace GuestABI
@@ -81,7 +79,10 @@ struct Result<ABI, SyscallReturn,
     static void
     store(ThreadContext *tc, const SyscallReturn &ret)
     {
-        tc->setIntReg(ABI::ReturnValueReg, ret.encodedValue());
+        if (ret.suppressed() || ret.needsRetry())
+            return;
+
+        tc->setIntReg(X86ISA::INTREG_RAX, ret.encodedValue());
     }
 };