[gdb/exp] Fix gdb.fortran/intrinsics.exp fail on arm

When running test-case gdb.fortran/intrinsics.exp on arm-linux, I get:
...
(gdb) p cmplx (4,4,16)^M
/home/linux/gdb/src/gdb/f-lang.c:1002: internal-error: eval_op_f_cmplx: \
  Assertion `kind_arg->code () == TYPE_CODE_COMPLEX' failed.^M
A problem internal to GDB has been detected,^M
further debugging may prove unreliable.^M
----- Backtrace -----^M
FAIL: gdb.fortran/intrinsics.exp: p cmplx (4,4,16) (GDB internal error)
...

The problem is that 16-byte floats are unsupported:
...
$ gfortran test.f90
test.f90:2:17:

    2 |     REAL(kind=16) :: foo = 1
      |                 1
Error: Kind 16 not supported for type REAL at (1)
...
and consequently we end up with a builtin_real_s16 and builtin_complex_s16 with
code TYPE_CODE_ERROR.

Fix this by bailing out asap when encountering such a type.

Without this patch we're able to do the rather useless:
...
(gdb) ptype real*16
type = real*16
(gdb) ptype real_16
type = real*16
...
but with this patch we get:
...
(gdb) ptype real*16
unsupported kind 16 for type real*4
(gdb) ptype real_16
unsupported type real*16
...

Tested on arm-linux.

PR fortran/30537
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30537