[rust] Link dylibs with LTO

This currently only applies to libstd, and automatically to future
dylibs introduced to the build.

We start to properly set `-Zdylib-lto` since https://fxrev.dev/1001661.

Context on linking dylibs in LTO:
rust-lang/rust#31854 (comment)

Bug: 394439548

Change-Id: I48237a739cb1b2bd02c284306d6e82d43eb83cc0
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1192917
Commit-Queue: Jay Zhuang <[email protected]>
Reviewed-by: David Turner <[email protected]>
Reviewed-by: Gulfem Savrun Yeniceri <[email protected]>

Author: jayzhuang

Diff for: build/config/BUILDCONFIG.gn

@@ -3018,19 +3018,6 @@ template("_fuchsia_shared_library") {
     _link_output_dir =
         rebase_path(get_path_info(link_output_file, "dir"), root_build_dir)
 
-    # rustc doesn't support building dylibs with LTO enabled without the
-    # `-Zdylib-lto` flag. For now we just disable LTO on these targets.
-    if (defined(configs) && defined(crate_type)) {
-      configs += [
-        "//build/config/lto",
-        "//build/config/lto:thinlto",
-      ]
-      configs -= [
-        "//build/config/lto",
-        "//build/config/lto:thinlto",
-      ]
-    }
-
     # Note: the manifest-related metadata is added to the builder target as
     # some targets (e.g. drivers) depend on it directly in the builder
     # toolchain.

Diff for: examples/rust/dylib/rust-bin/BUILD.gn

@@ -21,11 +21,6 @@ rustc_test("rust-bin") {
     "//examples/rust/dylib/rust-shared",
     "//src/lib/fuchsia",
   ]
-
-  # rustc doesn't support linking to dylibs while using LTO unless the
-  # `-Zdylib-lto` flag is passed. For now we exclude this binary from building
-  # with any LTO variants.
-  exclude_toolchain_tags = [ "lto" ]
 }
 
 fuchsia_unittest_package("rust-shared") {

Diff for: src/lib/ui/carnelian/BUILD.gn

@@ -129,9 +129,6 @@ rustc_library("carnelian") {
   }
 
   configs += [ ":carnelian-performance" ]
-
-  # Dynamic linking libstd is incompatible with LTO.
-  configs -= [ "//build/config/fuchsia:dynamic_rust_standard_library" ]
 }
 
 config("carnelian-performance") {