driver: Forward '-lgfortran', '-lm' to offloading compilation

driver: Forward '-lgfortran', '-lm' to offloading compilation

..., so that users don't manually need to specify
'-foffload-options=-lgfortran', '-foffload-options=-lm' in addition to
'-lgfortran', '-lm' (specified manually, or implicitly by the driver).

	gcc/
	* gcc.cc (driver_handle_option): Forward host '-lgfortran', '-lm'
	to offloading compilation.
	* config/gcn/mkoffload.cc (main): Adjust.
	* config/nvptx/mkoffload.cc (main): Likewise.
	* doc/invoke.texi (foffload-options): Update example.
	libgomp/
	* testsuite/libgomp.fortran/fortran.exp (lang_link_flags): Don't
	set.
	* testsuite/libgomp.oacc-fortran/fortran.exp (lang_link_flags):
	Likewise.
	* testsuite/libgomp.c/simd-math-1.c: Remove
	'-foffload-options=-lm'.
	* testsuite/libgomp.fortran/fortran-torture_execute_math.f90:
	Likewise.
	* testsuite/libgomp.oacc-fortran/fortran-torture_execute_math.f90:
	Likewise.