Fix gimplification of ordering comparisons of arrays of bytes

Fix gimplification of ordering comparisons of arrays of bytes

The Ada compiler now defers to the gimplifier for ordering comparisons of
arrays of bytes (Ada parlance for <, >, <= and >=) because the gimplifier
in turn defers to memcmp for them, which implements the required semantics.

However, the gimplifier has a special processing for aggregate types whose
mode is not BLKmode and this processing deviates from the memcmp semantics
when the target is little-endian.

gcc/
	* gimplify.cc (gimplify_scalar_mode_aggregate_compare): Add support
	for ordering comparisons.
	(gimplify_expr) <default>: Call gimplify_scalar_mode_aggregate_compare
	only for integral scalar modes.

gcc/testsuite/
	* gnat.dg/array42.adb, gnat.dg/array42_pkg.ads: New test.