[testsuite] test for weak_undefined support and add options

A number of tests that depend on weak undefined symbols fail to
require that support, and arrange to skip some targets and add special
options to others on a test-by-test basis.  Fix this by stating the
requirement explicitly, and adding a proc to return any required
options.

Others rely on weak undefined symbols only to test for the
availability of posix_memalign.  Drop that in favor of dg effective
target support for posix_memalign.


for  gcc/ChangeLog

	* doc/sourcebuild.texi (weak_undefined, posix_memalign):
	Document options and effective targets.

for  gcc/testsuite/ChangeLog

	* lib/target-supports.exp (add_options_for_weak_undefined):
	New.
	(check_effective_target_weak_undefined): Use it.
	(check_effective_target_posix_memalign): New.
	* gcc.dg/torture/pr53922.c: Drop skips and custom options in
	favor of effective target requirement and added options for
	weak_undefined symbols.
	* gcc.dg/torture/pr90020.c: Likewise.
	* gcc.dg/addr_equal-1.c: Likewise.
	* gcc.target/aarch64/aapcs64/aapcs64.exp: Likewise, for
	abitest.S-using tests.
	* gcc.dg/torture/pr60092.c: Likewise, but in favor of
	posix_memalign tests.
	* gcc.dg/vect/vect-tail-nomask-1.c: Likewise.