bash: Add a toolchain for local Bash.

Bazel automatically detects the local Bash and
creates a custom toolchain rule for it.

Later, rules that use Bash will require this
toolchain and retrieve Bash's path from it instead
of relying on hardcoded paths or the
`--shell_executable` flag.

See https://github.com/bazelbuild/bazel/issues/4319

Change-Id: Idd8242a20d202b1f5a56cddac95b625c6c08ede9

Closes #4980.

Change-Id: Ic2406a4da260b284e15852070d58472ca18340af
PiperOrigin-RevId: 193022708
diff --git a/tools/BUILD b/tools/BUILD
index 21a95fc..0b5835a 100644
--- a/tools/BUILD
+++ b/tools/BUILD
@@ -31,6 +31,7 @@
         "//tools/test:srcs",
         "//tools/python:srcs",
         "//tools/runfiles:srcs",
+        "//tools/sh:srcs",
         "//tools/whitelists:srcs",
         "//tools/zip:srcs",
     ],
@@ -62,6 +63,7 @@
         "//tools/test:srcs",
         "//tools/osx/crosstool:srcs",
         "//tools/osx:srcs",
+        "//tools/sh:embedded_tools",
         "//tools/whitelists:srcs",
         "//tools/zip:srcs",
     ],