rust: force crate hash to use bitbake input hash

This should eliminate the problems that have been seen when bitbake
rebuilds part of the rust stack (due to missing sstate) and ends up
getting hash mismatches.  Unlike rust's internally generated hash, ours
is guaranteed to be stable.

classes/rust.bbclass

@@ -1,7 +1,7 @@
 RUSTC = "rustc"
 
 # FIXME: --sysroot might be needed
-RUSTC_ARCHFLAGS += "--target=${TARGET_SYS} -C rpath"
+RUSTC_ARCHFLAGS += "--target=${TARGET_SYS} -C rpath -C crate_hash=${BB_TASKHASH}"
 
 def rust_base_dep(d):
     # Taken from meta/classes/base.bbclass `base_dep_prepend` and modified to