fortran: Add -fcoarray=shared option to auto-link -lcaf_shmem

fortran: Add -fcoarray=shared option to auto-link -lcaf_shmem

The new -fcoarray=shared option provides a convenient shorthand for
the common invocation -fcoarray=lib -lcaf_shmem.  The driver transforms
-fcoarray=shared into -fcoarray=lib for the frontend and automatically
appends -lcaf_shmem to the link command.  Existing uses of -fcoarray=lib
are unaffected.

Assisted by: Claude Sonnet 4.6

gcc/:

	* flag-types.h (gfc_fcoarray): Add GFC_FCOARRAY_SHARED.

gcc/fortran/:

	* lang.opt (fcoarray=): Add shared enum value; update help text.
	* gfortranspec.cc (CAF_SHMEM_LIBRARY): New macro.
	(lang_specific_driver): Detect -fcoarray=shared in first pass and
	set need_caf_shmem flag.  In second pass, transform -fcoarray=shared
	to -fcoarray=lib for cc1.  Append -lcaf_shmem when need_caf_shmem
	is set and linking is active.

gcc/testsuite/:

	* gfortran.dg/coarray_51.f90: New test.