Add support for --@bazel_tools//tools/cpp:link_extra_lib (#57)

If you want to use shared sanitizer libraries for mojo tests this is
required so that the final mojo binary has the rpath to the library set
correctly and it's available at runtime.

Author: keith

mojo/private/mojo_binary_test.bzl

@@ -41,6 +41,15 @@ then be used in copts with the $(location) function.
     "_mojo_copts": attr.label(
         default = Label("//:mojo_copt"),
     ),
+    "_link_extra_lib": attr.label(
+        default = "@bazel_tools//tools/cpp:link_extra_lib",
+        providers = [CcInfo],
+        doc = """\
+Pull in extra libraries passed with @bazel_tools//tools/cpp:link_extra_lib.
+This is useful for shared sanitizer libraries which need to have rpaths added
+by bazel.
+""",
+    ),
 }
 
 _TOOLCHAINS = use_cpp_toolchain() + [
@@ -95,7 +104,8 @@ def _mojo_binary_test_implementation(ctx, *, shared_library = False):
         if not file.dirname.startswith(root_directory):
             args.add("-I", file.dirname)
 
-    import_paths, transitive_mojopkgs = collect_mojoinfo(ctx.attr.deps + mojo_toolchain.implicit_deps)
+    all_deps = ctx.attr.deps + mojo_toolchain.implicit_deps + ([ctx.attr._link_extra_lib] if ctx.attr._link_extra_lib else [])
+    import_paths, transitive_mojopkgs = collect_mojoinfo(all_deps)
     args.add_all(import_paths, before_each = "-I")
 
     # NOTE: Argument order:
@@ -166,7 +176,7 @@ def _mojo_binary_test_implementation(ctx, *, shared_library = False):
         actions = ctx.actions,
         feature_configuration = feature_configuration,
         cc_toolchain = cc_toolchain,
-        linking_contexts = [object_linking_context] + [dep[CcInfo].linking_context for dep in (ctx.attr.deps + mojo_toolchain.implicit_deps) if CcInfo in dep],
+        linking_contexts = [object_linking_context] + [dep[CcInfo].linking_context for dep in all_deps if CcInfo in dep],
         name = ctx.label.name,
         user_link_flags = ctx.attr.linkopts,
         **link_kwargs
@@ -183,7 +193,7 @@ def _mojo_binary_test_implementation(ctx, *, shared_library = False):
     # Collect transitive shared libraries that must exist at runtime
     python_imports = []
     transitive_libraries = []
-    for target in ctx.attr.deps + mojo_toolchain.implicit_deps:
+    for target in all_deps:
         transitive_runfiles.append(target[DefaultInfo].default_runfiles)
 
         if PyInfo in target: