libpayload: x86/exec - fix return value passing

The pointer to write the return value to is in %ecx, not %eax. Writing
to (%eax) leads to memory corruptions as %eax holds the return value,
e.g. would write zero to address zero for a "successful" returning
payload.

Change-Id: I82df27ae89a9e3d25f479ebdda2b50ea57565459
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Reviewed-on: https://review.coreboot.org/18332
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

payloads/libpayload/arch/x86/exec.S

@@ -85,7 +85,7 @@ i386_do_exec:
          */
 
 	movl 20(%ebp), %ecx
-	movl %eax, (%eax)
+	movl %eax, (%ecx)
 
 	/* Get the rest of the saved registers */
 	popl %ecx