gdb: move xcr0 == 0 check into i386_linux_core_read_description

Currently, in i386_linux_core_read_description, if GDB fails to
extract an xcr0 value from the core file, then we will have a default
zero value for the xcr0 variable, we still call the
i386_linux_read_description function, which checks for this zero value
and returns nullptr.

Back in i386_linux_core_read_description we spot the nullptr return
value from i386_linux_read_description and call
i386_linux_read_description again, but this time passing a default
value for xcr0.

In the next commit I plan to rework i386_linux_read_description, and
in so doing I will remove the check for xcr0 == 0, this is inline with
how the amd64 code is written.

However, this means that the 'xcr0 == 0' check needs to move up the
stack to i386_linux_core_read_description, again, this brings the i386
code into line with the amd64 code.

This is just a refactor in preparation for the next commit, there
should be no user visible changes after this commit.

Approved-By: Felix Willgerodt <felix.willgerodt@intel.com>
Approved-By: John Baldwin <jhb@FreeBSD.org>