[Ada] GNAT.Expect (Expect_Internal): Try to call 'poll' few times

[Ada] GNAT.Expect (Expect_Internal): Try to call 'poll' few times

'poll' returns -1 in case of any error (including interruption by a
signal), so call need to be repeated few times to avoid false failures.

2019-09-17  Vadim Godunko  <godunko@adacore.com>

gcc/ada/

	* libgnat/g-expect.adb (Expect_Internal): Try to call 'poll' few
	times.

From-SVN: r275782