libgcov: use proper type for n_functions

gcov_info::n_functions type is initialized by generated
code in build_info_type:

/* n_functions */
field = build_decl (BUILTINS_LOCATION, FIELD_DECL, NULL_TREE,
		    get_gcov_unsigned_t ());

It uses gcov_unsigned_t, but the struct definition in libgcov.h uses
unsigned type. That brings troubled on 16-bit targets.

	PR gcov-profile/105535

libgcc/ChangeLog:

	* libgcov.h (struct gcov_info): Use gcov_unsigned_t for
	n_functions.

Co-Authored-By: Hans-Peter Helfert <peter-helfert@t-online.de>