Add support for benchmarking

[dbittman]

This PR adds support for the unstable bench framework that's part of rust's test system. The following changes were made:

1. Add a --benches flag to xtask's start-qemu command. This works similar to --tests, but sets up the test binaries to be invoked for benchmarking.
2. Modifies rust to add support for the math library (benchmarks need math functions, we don't link against libm by default because we don't have libc, so I had to bump compiler_builtins to a version that exposes its math library).

With this PR, a programmer can add #[bench] attributes to test functions and use the benchmarking system. These then get run as part of the test system if --benches is supplied. The results are, for now, only printed, and not used as part of CI.

[gvnn3]

Seems straightforward. Merge it in.