Fix Rust testing

Fix Rust testing

This changes the gdb test suite to omit -fno-stack-protector when
compiling Rust code.  This makes Rust testing work again.

I think I saw this patch somewhere already, but I couldn't find it
again just now, so I'm checking this version in.

gdb/testsuite/ChangeLog
2019-04-24  Tom Tromey  <tromey@adacore.com>

	* lib/gdb.exp (gdb_compile): Don't add -fno-stack-protector for
	Rust.