Implement -fexternal-blas64 option.

Implement -fexternal-blas64 option.

Libraries like Intel MKL use 64-bit integers in their API, but gfortran
up to now only provides external BLAS for matmul with 32-bit
integers.  This straightforward patch provides a new option -fexternal-blas64
to remedy that situation.

gcc/fortran/ChangeLog:

	* frontend-passes.cc (optimize_namespace): Handle
	flag_external_blas64.
	(call_external_blas): If flag_external_blas is set, use
	gfc_integer_4_kind as the argument kind, gfc_integer_8_kind otherwise.
	* gfortran.h (gfc_integer_8_kind): Define.
	* invoke.texi: Document -fexternal-blas64.
	* lang.opt: Add -fexternal-blas64.
	* lang.opt.urls: Regenerated.
	* options.cc (gfc_post_options): -fexternal-blas is incompatible
	with -fexternal-blas64.

gcc/testsuite/ChangeLog:

	* gfortran.dg/matmul_blas_3.f90: New test.