rust-std: Add support for thread_local_key

[flaub]

From discord:

Currently, any crate which depends on once_cell (including std::collections::HashMap) will compile but experience a runtime panic because the method thread_local_key is unimplemented.

Exception { what: "panicked at 'should not be used on this target', /risc0-rust-starter/target/debug/build/methods-e1d0c7ab6d906214/out/rust-std_5394976e3be5c2fbe950f41f75eb392cd888657933629544e0a55a75bb0e59d9/library/std/src/sys/unsupported/thread_local_key.rs:25:5" }'

Since the zkvm is single-threaded, it seems like this should be relatively straightforward.

[shkoo]

Should be fixed by #216