Mark wasm32 `unreachable` as safe (#905)

The function was recently marked as safe:
https://doc.rust-lang.org/nightly/core/arch/wasm32/fn.unreachable.html.

>This function is safe to call and immediately aborts the execution.
22 jobs for master in 10 minutes and 3 seconds (queued for 10 seconds)
Name Stage Failure
failed
test Workspace
test src/lib.rs - SpreadLayout (line 70) ... ok
test src/lib.rs - StorageLayout (line 139) ... ok
test src/lib.rs - PackedLayout (line 103) ... ok

test result: ok. 4 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.59s

error: test failed, to rerun pass '-p ink_lang_macro --test compile_tests'
Cleaning up file based variables
ERROR: Job failed: exit code 1