[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/.bazelrc

@@ -0,0 +1 @@
+build --incompatible_enable_cc_toolchain_resolution

bazel/test_secondary_lto_cache/.gitignore

@@ -0,0 +1,4 @@
+bazel-bin
+bazel-out
+bazel-test_secondary_lto_cache
+bazel-testlogs

bazel/test_secondary_lto_cache/BUILD

@@ -0,0 +1,19 @@
+load("@emsdk//emscripten_toolchain:wasm_rules.bzl", "wasm_cc_binary")
+
+cc_binary(
+    name = "hello-world",
+    srcs = ["hello-world.cc"],
+    linkopts = [
+        "-sAUTO_NATIVE_LIBRARIES=0",
+        "-flto",
+    ],
+)
+
+wasm_cc_binary(
+    name = "hello-world-wasm",
+    cc_target = ":hello-world",
+    outputs = [
+        "hello-world.js",
+        "hello-world.wasm",
+    ],
+)

bazel/test_secondary_lto_cache/MODULE.bazel

@@ -0,0 +1 @@
+bazel_dep(name = "platforms", version = "0.0.9")

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"
+    ]
+})

bazel/test_secondary_lto_cache/hello-world.cc

@@ -0,0 +1,6 @@
+#include <iostream>
+
+int main(int argc, char** argv) {
+  std::cout << "hello world!" << std::endl;
+  return 0;
+}