Tools like GNATprove built as GNAT backends rely on adabkend.adb to
handle generic switches like the optimisation switches -Oxxx.
This patch adds support for -Og and -Ofast that was missing.

There is no impact on compilation.

2019-07-05  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* adabkend.adb (Scan_Back_End_Switches): Accept -Og and -Ofast
	switches.

From-SVN: r273108