Apply extra_rustc_flags list to all toolchains (#2693)

Fixes #2692 

This fixes `rust_register_toolchains` to correctly call
`rust_toolchain_repository` with a list of `extra_rustc_flags`.

Previously, the `extra_rustc_flags` argument of
`rust_register_toolchains` was assumed (when set) to be a dict of
toolchain triple to list. With this change, a `list` parameter is used
for all toolchain triples.

Co-authored-by: UebelAndre <github@uebelandre.com>

rust/repositories.bzl

@@ -1087,6 +1087,16 @@ def rust_repository_set(
         elif type(extra_target_triples) == "dict":
             target_compatible_with = extra_target_triples.get(toolchain.target_triple)
 
+        # Infer toolchain-specific rustc flags depending on the type (list, dict, optional) of extra_rustc_flags
+        if extra_rustc_flags == None:
+            toolchain_extra_rustc_flags = []
+        elif type(extra_rustc_flags) == "list":
+            toolchain_extra_rustc_flags = extra_rustc_flags
+        elif type(extra_rustc_flags) == "dict":
+            toolchain_extra_rustc_flags = extra_rustc_flags.get(toolchain.target_triple)
+        else:
+            fail("extra_rustc_flags should be a list or a dict")
+
         all_toolchain_names.append(rust_toolchain_repository(
             name = toolchain.name,
             allocator_library = allocator_library,
@@ -1099,7 +1109,7 @@ def rust_repository_set(
             edition = edition,
             exec_triple = exec_triple,
             extra_exec_rustc_flags = extra_exec_rustc_flags,
-            extra_rustc_flags = extra_rustc_flags.get(toolchain.target_triple) if extra_rustc_flags != None else None,
+            extra_rustc_flags = toolchain_extra_rustc_flags,
             opt_level = opt_level.get(toolchain.target_triple) if opt_level != None else None,
             target_settings = target_settings,
             iso_date = toolchain.channel.iso_date,