[gdb/testsuite] Fix charlen type in mixed-lang-stack.c

In gdb.fortran/mixed-lang-stack.f90, we have fortran function mixed_func_1d:
...
subroutine mixed_func_1d(a, b, c, d, str)
  use, intrinsic :: iso_c_binding, only: c_int, c_float, c_double
  use, intrinsic :: iso_c_binding, only: c_float_complex
  implicit none
  integer(c_int) :: a
  real(c_float) :: b
  real(c_double) :: c
  complex(c_float_complex) :: d
  character(len=*) :: str
...
which we declare in C in gdb.fortran/mixed-lang-stack.c like this:
...
extern void mixed_func_1d_ (int *, float *, double *, complex float *,
                            char *, size_t);
...

The fortran string parameter str is passed as a char *, and an additional
argument str_ for the string length.  The type used for the string length
argument is size_t, but for gcc 7 and earlier, the actual type is int
instead ( see
https://gcc.gnu.org/onlinedocs/gfortran/Argument-passing-conventions.html ).

Fix this by declaring the string length type depending on the gcc version:
...
 #if !defined (__GNUC__) || __GNUC__ > 7
 typedef size_t fortran_charlen_t;
 #else
 typedef int fortran_charlen_t;
...

Tested on x86_64-linux, with gcc-7 and gcc-8.

gdb/testsuite/ChangeLog:

2020-08-15  Tom de Vries  <tdevries@suse.de>

	* gdb.fortran/mixed-lang-stack.c (fortran_charlen_t): New type.
	(mixed_func_1d_): Use fortran_charlen_t in decl.