Add explicit search path for linker on macOS.

Linking on macOS is invoked via clang++. If clang++ cannot find ld on the
default search path, clang++ will invoke xcrun to search for ld. This CL
explicitly passes the path to ld if we're using hermetic xcode. This way clang++
will not invoke xcrun.

Bug: 971452
Change-Id: I5bcaa577d9ccc6d3ceca6e2524482e299b371e92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1649179Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667154}

build/toolchain/mac/BUILD.gn

@@ -125,6 +125,10 @@ template("mac_toolchain") {
     cxx = compiler_prefix + _cxx
     ld = _cxx
 
+    # Set the explicit search path for clang++ so it uses the right linker
+    # binary.
+    ld += " -B " + mac_bin_path
+
     if (defined(toolchain_args.coverage_instrumentation_input_file)) {
       toolchain_coverage_instrumentation_input_file =
           toolchain_args.coverage_instrumentation_input_file