[bazel] Use binary_alias instead of symlinks for ld.lld and friends

[anguslees]

Summary

Symlinks are poorly supported in Bazel+remote-exec. Create the links from ld.lld,
ld64.lld, lld-link, wasm-ld to lld using LLVM's binary_alias instead.

[anguslees]

(I don't have LLVM commit access, please merge for me when appropriate)

[MaskRay]

Looks great!

utils/bazel/llvm-project-overlay/lld/BUILD.bazel

 # This file is licensed under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 load("@bazel_skylib//rules:expand_template.bzl", "expand_template")
 load("//llvm:tblgen.bzl", "gentbl")
+load("//llvm:binary_alias.bzl", "binary_alias")
 load(
     "//:vars.bzl",
     "LLVM_VERSION",
 )
 
 package(
     default_visibility = ["//visibility:public"],
     features = ["layering_check"],

 )
 
 # These are the required names for lld running under different environs.
 #
 # Unix/Linux require that the binary be named "ld.lld".
 # macOS require that the binary be named "ld64.lld".
 # Windows require that the binary be named "lld-link".
 # WebAssembly builds require that the binary be named "wasm-ld".
-genrule(
-    name = "gen_lld",
-    srcs = [":lld"],
-    outs = [
+[
+    binary_alias(
+        name = name,
+        binary = ":lld",
+    )
+    for name in [
         "ld.lld",
         "ld64.lld",
         "lld-link",
         "wasm-ld",
-    ],
-    cmd =
-        "target=$$(basename $<); for n in ld.lld ld64.lld lld-link wasm-ld; do ln -sf $$target $(@D)/$$n; done",
-    output_to_bindir = 1,
-)
+    ]
+]