fix gamma symbol for static linking and new targets

The lgamma compat code is no longer built for !LIBM_SVID_COMPAT targets,
but the legacy gamma, gammaf and gammal symbols should be still defined,
so make them aliases to the non-compat lgamma code.

	* math/w_lgamma.c: New file.
	* math/w_lgammaf.c: New file.
	* math/w_lgammal.c: New file.