Fix gdb.python/py-thrhandle.exp failures for -m32 multilib

Fix gdb.python/py-thrhandle.exp failures for -m32 multilib

This patch fixes the following failures when testing with
"target_board unix/-m32" using a x86_64-pc-linux-gnu native GDB.

FAIL: gdb.python/py-thrhandle.exp: print thread for bogus handle thrs[3]
FAIL: gdb.python/py-thrhandle.exp: print thread for bogus handle thrs[4]
FAIL: gdb.python/py-thrhandle.exp: print thread id for thrs[0]
FAIL: gdb.python/py-thrhandle.exp: print thread id for thrs[1]
FAIL: gdb.python/py-thrhandle.exp: print thread id for thrs[2]
FAIL: gdb.python/py-thrhandle.exp: thread 0: fetch thread handle from thread
FAIL: gdb.python/py-thrhandle.exp: thread 0: verify that handles are the same
FAIL: gdb.python/py-thrhandle.exp: thread 1: fetch thread handle from thread
FAIL: gdb.python/py-thrhandle.exp: thread 1: verify that handles are the same
FAIL: gdb.python/py-thrhandle.exp: thread 2: fetch thread handle from thread
FAIL: gdb.python/py-thrhandle.exp: thread 2: verify that handles are the same

I've written it so that it might work for other 64-bit host / 32-bit target
combos too.

gdb/ChangeLog:

	* linux-thread-db.c (thread_db_target::thread_handle_to_thread_info):
	Add case for debugging 32-bit target on 64-bit host.  Revise
	comment.