Adds a new test directive COMPILABLE_MATH_TEST, and support has been
added for it in gdc-convert-test so that they are skipped if phobos is
not present on the target.

Only change in D runtime is a small documentation fix.

Reviewed-on: https://github.com/dlang/druntime/pull/3067
	     https://github.com/dlang/dmd/pull/11060

gcc/testsuite/ChangeLog:

	PR d/89418
	* lib/gdc-utils.exp (gdc-convert-test): Add dg-skip-if for compilable
	tests that depend on the phobos standard library.