rust: strip whitespace from features

This whitespace in here causes rust to not be happy with the features
and throw warnings about a feature such as ' +sse3' being unsupported
when '+sse3' works just fine. Amazingly will cause rustc to abort when
you try to resolve the features to valid configs with the `--print` arg
to rustc.

(cherry picked from commit ce9949725c)

recipes-devtools/rust/rust.inc

@@ -251,6 +251,7 @@ def rust_gen_target(d, thing, wd):
     if thing is "TARGET":
         features = d.getVar('TARGET_LLVM_FEATURES', True) or ""
     features = features or d.getVarFlag('FEATURES', arch, True) or ""
+    features = features.strip()
 
     # build tspec
     tspec = {}