[rust] update to stable syntax for mangling version

The unstable syntax gives a warning on the current toolchain. The new
syntax is stable as of rust-lang/rust#90128

Change-Id: If2ff4bdbae68256b8e13e1d2c54c6346aaac99f2
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/635923
Fuchsia-Auto-Submit: Dan Johnson <[email protected]>
Reviewed-by: Tyler Mandry <[email protected]>
Commit-Queue: Auto-Submit <[email protected]>

Author: ComputerDruid

build/config/rust/BUILD.gn

@@ -105,7 +105,7 @@ config("panic_abort") {
 
 config("v0_symbol_mangling") {
   if (rust_v0_symbol_mangling) {
-    rustflags = [ "-Zsymbol-mangling-version=v0" ]
+    rustflags = [ "-Csymbol-mangling-version=v0" ]
   }
 }