Fix 'char' initialization, copy, check in 'libgomp.oacc-fortran/acc-memcpy.f90'

Our dear friend '-Wuninitialized' reported:

    [...]/libgomp.oacc-fortran/acc-memcpy.f90:18:27:

       18 |     char(j) = int (j, int8)
          |                           ^
    Warning: ‘j’ may be used uninitialized [-Wmaybe-uninitialized]
    [...]/libgomp.oacc-fortran/acc-memcpy.f90:14:20:

       14 |   integer(int8) :: j
          |                    ^
    note: ‘j’ was declared here

..., but actually there were other issues.

	libgomp/
	* testsuite/libgomp.oacc-fortran/acc-memcpy.f90: Fix 'char'
	initialization, copy, check.