checkasm: x86: properly save rdx/edx in checked_call()

If the return value doesn't fit in a single register rdx/edx can in some
cases be used in addition to rax/eax.

Doesn't affect any of the existing checkasm tests but might be useful later.

Also comment the relevant code a bit better.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

tests/checkasm/x86/checkasm.asm

@@ -145,10 +145,15 @@ cglobal checked_call, 2,15,16,max_args*8+8
     or  r14, r5
 %endif
 
+    ; Call fail_func() with a descriptive message to mark it as a failure
+    ; if the called function didn't preserve all callee-saved registers.
+    ; Save the return value located in rdx:rax first to prevent clobbering.
     jz .ok
     mov  r9, rax
+    mov r10, rdx
     lea  r0, [error_message]
     call fail_func
+    mov rdx, r10
     mov rax, r9
 .ok:
     RET
@@ -182,9 +187,11 @@ cglobal checked_call, 1,7
     or   r3, r5
     jz .ok
     mov  r3, eax
+    mov  r4, edx
     lea  r0, [error_message]
     mov [esp], r0
     call fail_func
+    mov  edx, r4
     mov  eax, r3
 .ok:
     add  esp, max_args*4