Use ACX_NONCANONICAL_TARGET in gdb/configure

Shahab Vahedi pointed out that the patch to remove
gdb/testsuite/configure regressed the site.exp creation a bit -- it
left an unresolved configure substitution.  Andrew Burgess pointed out
that the patch removed the call to ACX_NONCANONICAL_TARGET, which
caused this problem.

This patch adds ACX_NONCANONICAL_TARGET to gdb's configure, and fixes
the bug.

gdb/ChangeLog
2021-06-05  Tom Tromey  <tromey@adacore.com>

	* configure: Rebuild.
	* configure.ac: Add ACX_NONCANONICAL_TARGET.