Fix internal error on anonymous access type equality

This is a small regression present on the mainline and 13 branch, in the
form of an internal error in gigi on anonymous access type equality.  We
now need to also accept them for anonymous access types that point to
compatible object subtypes in the language sense.

gcc/ada/
	* gcc-interface/utils2.cc (build_binary_op) <EQ_EXPR>: Relax
	assertion for regular pointer types.

gcc/testsuite/
	* gnat.dg/specs/anon4.ads: New test.