Update xfail pattern in gdb.rust/simple.exp

Update xfail pattern in gdb.rust/simple.exp

In PR rust/26197, Tom de Vries notes that the variant part rewrite
caused some regressions for the Rust compiler he has.  This compiler
is unusual in that it combines a relatively recent rustc with a
relatively old LLVM -- so variant parts are not emitted using DWARF.

Most of the bugs in that PR were already fixed by earlier patches, but
some lingered.  After some research we found that some of these never
did work -- which is consistent with the investigations we did into
the debug info -- but instead were xfail'd.  This patch updates the
xfails to cope with the new output.  (After this, just one failure
remains.)

Tom de Vries tested this using his rustc and suggested a fix that
appears in this version.

gdb/testsuite/ChangeLog
2020-08-17  Tom de Vries  <tdevries@suse.de>
	    Tom Tromey  <tromey@adacore.com>

	PR rust/26197:
	* gdb.rust/simple.exp (xfail_pattern): Update for new failure.