Fix timeout for gdb.xml/tdesc-reload.exp

For aarch64, the "info all-registers" output is very verbose.  If you run
this test using read1, it will timeout before the command output is done being
read.

Read line-by-line so we don't run into timeouts.

gdb/testsuite/ChangeLog:

2021-04-27  Luis Machado  <luis.machado@linaro.org>

	* gdb.xml/tdesc-reload.exp: Pass -lbl.