arch/x86/idt.S: Use 'iretq' for ENV_X86_64

Fix the warning below when building GA-945GCM-S2L with 64-bit:
src/arch/x86/idt.S:216: Warning: no instruction mnemonic suffix given and no register operands; using default for `iret'

Change-Id: Ibbc106714e25293951a71d84fea0a660f41f9c02
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/61336
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/arch/x86/idt.S

@@ -170,6 +170,7 @@ int_hand:
 	pop	%r15
 
 	add	$16, %rsp /* pop of the vector and error code */
+	iretq
 #else
 	/* At this point, on x86-32, on the stack there is:
 	 *  0(%esp) vector
@@ -211,6 +212,5 @@ int_hand:
 	popl	%edi
 
 	addl	$8, %esp /* pop of the vector and error code */
-#endif
-
 	iret
+#endif