libgomp/nvptx: Prepare for reverse-offload callback handling, resolve spurious SIGSEGVs

libgomp/nvptx: Prepare for reverse-offload callback handling, resolve spurious SIGSEGVs

Per commit r13-3460-g131d18e928a3ea1ab2d3bf61aa92d68a8a254609
"libgomp/nvptx: Prepare for reverse-offload callback handling",
I'm seeing a lot of libgomp execution test regressions.  Random
example, 'libgomp.c-c++-common/error-1.c':

    [...]
      GOMP_OFFLOAD_run: kernel main$_omp_fn$0: launch [(teams: 1), 1, 1] [(lanes: 32), (threads: 8), 1]

    Thread 1 "a.out" received signal SIGSEGV, Segmentation fault.
    0x00007ffff793b87d in GOMP_OFFLOAD_run (ord=<optimized out>, tgt_fn=<optimized out>, tgt_vars=<optimized out>, args=<optimized out>) at [...]/source-gcc/libgomp/plugin/plugin-nvptx.c:2127
    2127            if (__atomic_load_n (&ptx_dev->rev_data->fn, __ATOMIC_ACQUIRE) != 0)
    (gdb) print ptx_dev
    $1 = (struct ptx_device *) 0x6a55a0
    (gdb) print ptx_dev->rev_data
    $2 = (struct rev_offload *) 0xffffffff00000000
    (gdb) print ptx_dev->rev_data->fn
    Cannot access memory at address 0xffffffff00000000

	libgomp/
	* plugin/plugin-nvptx.c (nvptx_open_device): Initialize
	'ptx_dev->rev_data'.