fix: Add missing assembly src files to ObjCxx (#1441)

Assembly files like `.S` are allowed in `srcs` of `objc_library` but we
were dropping these because it was only being handled in the `else` case
where we do not have Objective-C srcs.

A package that requires this change to build is:
https://github.com/microsoft/plcrashreporter. I did not add an example
of that here because it unfortunately still does not build in sandbox
mode (because of issues with the `Package.swift` using `excludes`, I can
work on updating the package and adding the example here at a later
time).

[See this change for how it was done before we split the
targets](6d12d53#diff-b4f84bf6654c556e5a44572cef98ce510f12dfea788eed39432b9af5f3e4559dL21)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: luispadron

swiftpkg/internal/swiftpkg_build_files.bzl

@@ -511,7 +511,8 @@ def _clang_target_build_file(repository_ctx, pkg_ctx, target):
                     name = child_name,
                     attrs = attrs,
                     rule_kind = rule_kind,
-                    srcs = clang_src_info.organized_srcs.cxx_srcs +
+                    srcs = clang_src_info.organized_srcs.assembly_srcs +
+                           clang_src_info.organized_srcs.cxx_srcs +
                            clang_src_info.organized_srcs.objcxx_srcs +
                            clang_src_info.organized_srcs.other_srcs +
                            res_objcxx_srcs,