RFR: JDK-8076629 - DIO UART read_start method may return before read mutex release

[alexey karaksin]

issue: https://bugs.openjdk.java.net/browse/JDK-8076629
webrev: http://cr.openjdk.java.net/~amironov/8076629/

Please review the changes made to fix the glitch in the javacall layer 
that allows
  javacall_uart_read_start exit before a native mutex release.

Regards,
Alexey