[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/test_secondary_lto_cache/WORKSPACE

@@ -0,0 +1,30 @@
+local_repository(
+    name = "emsdk",
+    path = "..",
+)
+
+load("@emsdk//:deps.bzl", "deps")
+
+deps()
+
+load("@emsdk//:emscripten_deps.bzl", "emscripten_deps")
+
+emscripten_deps()
+
+load("@emsdk//:toolchains.bzl", "register_emscripten_toolchains")
+
+register_emscripten_toolchains(cache = {
+    "configuration": ["--lto"],
+    "targets": [
+        "crtbegin",
+        "libprintf_long_double-debug",
+        "libstubs-debug",
+        "libnoexit",
+        "libc-debug",
+        "libdlmalloc",
+        "libcompiler_rt",
+        "libc++-noexcept",
+        "libc++abi-debug-noexcept",
+        "libsockets"
+    ]
+})