[Bazel] Support generating a secondary cache (#1405)

This is a working solution for generating a separate Emscripten cache.
Note that this requires an additional entry in the workspace as follows:
```starlark
load("@emsdk//:emscripten_cache.bzl", emsdk_emscripten_cache = "emscripten_cache")
emsdk_emscripten_cache()
```
When used like this, the default Emscripten cache will be used. However,
if the entry is as follows:
```starlark
load("@emsdk//:emscripten_cache.bzl", emsdk_emscripten_cache = "emscripten_cache")
emsdk_emscripten_cache(flags = ["--lto"])
```
Then embuilder will be called to build all system libraries and ports
(i.e., the `ALL` option to embuilder) with the LTO option enabled. This
can take awhile, so I have also made possible to specify which libraries
you want to build explicitly:
```starlark
load("@emsdk//:emscripten_cache.bzl", emsdk_emscripten_cache = "emscripten_cache")
emsdk_emscripten_cache(
    flags = ["--lto"],
    libraries = [
        "crtbegin",
        "libprintf_long_double-debug",
        "libstubs-debug",
        "libnoexit",
        "libc-debug",
        "libdlmalloc",
        "libcompiler_rt",
        "libc++-noexcept",
        "libc++abi-debug-noexcept",
        "libsockets"
    ]
)
```

Resolves #807, resolves #971, resolves #1099, resolves #1362, resolves
#1401

bazel/emscripten_toolchain/BUILD.bazel

@@ -5,7 +5,7 @@ package(default_visibility = ["//visibility:public"])
 filegroup(
     name = "common_files",
     srcs = [
-        "emscripten_config",
+        "@emscripten_cache//:emscripten_config",
         "env.sh",
         "env.bat",
         "@nodejs//:node_files",
@@ -60,7 +60,7 @@ cc_library(name = "malloc")
 emscripten_cc_toolchain_config_rule(
     name = "wasm",
     cpu = "wasm",
-    em_config = "emscripten_config",
+    em_config = "@emscripten_cache//:emscripten_config",
     emscripten_binaries = "@emsdk//:compiler_files",
     script_extension = select({
         "@bazel_tools//src/conditions:host_windows": "bat",