Update the core file architecture if a target description is present

Update the core file architecture if a target description is present

At the moment, the core target has its own gdbarch (m_core_gdbarch), and that
gets set from the core_bfd on the core target's constructor.

That gdbarch doesn't contain a target description because it is constructed
before we get a chance to fetch the target description.

As a result, some hooks that depend on the target description being set are
not set, and that leads to problems. One of the examples is
gdbarch_report_signal_info, which is used to show AArch64 tag violation
information.

Fix this by reading the target description before fetching the core file's
gdbarch.

gdb/ChangeLog:

2021-06-25  Luis Machado  <luis.machado@linaro.org>

	* corelow.c (core_target::core_target) Update to read target
	description.