[rust] Enable v0 symbol mangling by default

This was stabilized toward the beginning of 2022
(rust-lang/rust#90128), though it still isn't
the default used by the compiler. Fuchsia tooling should know how to
demangle Rust symbols by now (except for tools that already don't
work with the legacy mangling scheme, http://fxbug.dev/64315).

Test: Manually verified that zxdb can symbolize backtraces when this
option is enabled.

Fixed: 57302
Change-Id: If0284671499af04194ab6033a3b7c54cf2314cd2
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/769822
Reviewed-by: Joseph Ryan <josephry@google.com>
Commit-Queue: Tyler Mandry <tmandry@google.com>
Fuchsia-Auto-Submit: Tyler Mandry <tmandry@google.com>
Reviewed-by: David Koloski <dkoloski@google.com>
Reviewed-by: Dangyi Liu <dangyi@google.com>

Author: tmandry

build/config/rust/BUILD.gn

@@ -22,10 +22,7 @@ import("//build/toolchain/rbe.gni")
 declare_args() {
   # Controls whether the rust compiler uses v0 symbol mangling scheme
   # (see https://github.com/rust-lang/rfcs/blob/HEAD/text/2603-rust-symbol-name-mangling-v0.md).
-  # The v0 symbol mangling scheme requires upstream LLVM support when demangling,
-  # so it is not on by default.
-  # TODO(fxbug.dev/57302): Enable v0 mangling by default.
-  rust_v0_symbol_mangling = false
+  rust_v0_symbol_mangling = true
 
   # Enable incremental rust compilation. Takes a path to the directory to use
   # as the cache.