linux_spu_make_corefile_notes: return note_data instead of nullptr

linux_spu_make_corefile_notes: return note_data instead of nullptr

Since commit

  9018be2 ("Make target_read_alloc & al return vectors")

the test gdb.threads/gcore-stale-thread.exp test results in UNSUPPORTED:

  UNSUPPORTED: gdb.threads/gcore-stale-thread.exp: save a corefile

The problem is that the linux_spu_make_corefile_notes started returning
nullptr when reading TARGET_OBJECT_SPU fails.  The previous (and proper)
behaviour is to return the note_data received as a parameter, so that
other functions may continue to append to this buffer.

With this patch, the test goes back to PASS.

gdb/ChangeLog:

	* linux-tdep.c (linux_spu_make_corefile_notes): Return note_data
	instead of nullptr.