Update gdb.rust tests for Rust 1.49

Rust 1.49 was released today, and it includes some library changes
which caused some gdb.rust tests to fail.  This patch adapts the test
suite to the new output.  I also verified that this continues to work
with Rust 1.48.

gdb/testsuite/ChangeLog
2020-12-31  Tom Tromey  <tom@tromey.com>

	* gdb.rust/simple.exp: Update output for Rust 1.49.