[Ada] Improve detection of end of the process by GNAT.Expect

[Ada] Improve detection of end of the process by GNAT.Expect

'read' system call may be interrupted by signal with 'errno' is set to
EINTER. In this case, re-try a few times.

2019-08-21  Vadim Godunko  <godunko@adacore.com>

gcc/ada/

	* libgnat/g-expect.adb (Expect_Internal): Attempt to read
	several times when 'read' returns non-positive.

From-SVN: r274791