PR c/88172 - attribute aligned of zero silently accepted but ignored
PR testsuite/88208 - new test case c-c++-common/builtin-has-attribute-3.c in r266335 has multiple excess errors

gcc/ChangeLog:

	PR c/88172
	PR testsuite/88208
	* doc/extend.texi (attribute constructor): Clarify.

gcc/c/ChangeLog:

	PR c/88172
	PR testsuite/88208
	* c-decl.c (declspec_add_alignas): Adjust call to check_user_alignment.

gcc/c-family/ChangeLog:

	PR c/88172
	PR testsuite/88208
	* c-attribs.c (common_handle_aligned_attribute): Silently avoid setting
	alignments to values less than the target requires.
	(has_attribute): For attribute aligned consider both the attribute
	and the alignment bits.
	* c-common.c (c_init_attributes): Optionally issue a warning for
	zero alignment.

gcc/testsuite/ChangeLog:

	PR c/88172
	PR testsuite/88208
	* gcc.dg/attr-aligned-2.c: New test.
	* gcc.dg/builtin-has-attribute.c: Adjust.
	* c-c++-common/builtin-has-attribute-2.c: Same.
	* c-c++-common/builtin-has-attribute-3.c: Same.
	* c-c++-common/builtin-has-attribute-4.c: Same.
	* c-c++-common/builtin-has-attribute-5.c: New test.
	* gcc.target/aarch64/attr-aligned.c: Same.
	* gcc.target/i386/attr-aligned.c: Same.
	* gcc.target/powerpc/attr-aligned.c: Same.
	* gcc.target/sparc/attr-aligned.c: Same.

From-SVN: r266633