ada: Add switch to disable expansion of assertions in CodePeer mode

ada: Add switch to disable expansion of assertions in CodePeer mode

A new debug switch -gnatd_k is added, which has only effect in CodePeer
mode. When enabled, assertion expressions are no longer expanded (which
is the default in the CodePeer mode); instead, their expansion needs to
be explicitly enabled by pragma Assertion_Policy.

gcc/ada/

	* debug.adb (d_k): Use first available debug switch.
	* gnat1drv.adb (Adjust_Global_Switches): If new debug switch is
	active then don't expand assertion expressions by default.