This will reduce significantly the number of allocations done when
doing consecutive append operations.

2018-07-16  Nicolas Roche  <roche@adacore.com>

gcc/ada/

	* libgnat/a-strunb.adb, libgnat/a-strunb__shared.adb: Adjust growth
	factor from 1/32 to 1/2 for Unbounded_String.

From-SVN: r262697