Mozilla rustc check does not support crossbuild: let's remove it
Remove calls to unwrap_rustup, they fail if rustup isn't present

Do not try to figure out the rust target as it is broken on musl
instead use what is set in RUST_TARGET

Skip extra checks on cross builds (broken for arm*)

--- a/build/moz.configure/rust.configure	2020-12-12 01:35:33.000000000 +0100
+++ b/build/moz.configure/rust.configure	2020-12-15 19:45:16.467750787 +0100
@@ -91,9 +91,6 @@
     return unwrap
 
 
-rustc = unwrap_rustup(rustc, "rustc")
-cargo = unwrap_rustup(cargo, "cargo")
-
 
 set_config("CARGO", cargo)
 set_config("RUSTC", rustc)
@@ -260,7 +257,9 @@
         data.setdefault(key, []).append(namespace(rust_target=t, target=info))
     return data
 
-
+@imports('os')
+@imports(_from='mozbuild.util', _import='ensure_unicode')
+@imports(_from='mozbuild.util', _import='system_encoding')
 def detect_rustc_target(
     host_or_target, compiler_info, arm_target, rust_supported_targets
 ):
@@ -382,12 +381,12 @@
 
         return None
 
-    rustc_target = find_candidate(candidates)
+    rustc_target = os.environ['RUST_TARGET']
 
     if rustc_target is None:
         die("Don't know how to translate {} for rustc".format(host_or_target.alias))
 
-    return rustc_target
+    return ensure_unicode(rustc_target, system_encoding)
 
 
 @imports("os")
@@ -486,6 +485,7 @@
         host, compiler_info, arm_target, rust_supported_targets
     )
     if rustc_target != rustc_host:
+        return rustc_host
         if host.alias == rustc_target:
             configure_host = host.alias
         else:
@@ -507,7 +507,6 @@
                 )
             )
         )
-    assert_rust_compile(host, rustc_target, rustc)
     return rustc_target


@@ -521,7 +520,6 @@
     rustc_target = detect_rustc_target(
         target, compiler_info, arm_target, rust_supported_targets
     )
-    assert_rust_compile(target, rustc_target, rustc)
     return rustc_target