Revert "[build] Omit the vendor component in Fuchsia target triple"

This reverts commit cd5207a2a3d8a2a7f4e9579405f7323003cfae13.

Reason for revert: The new version of Rust toolchain that's using the shorter triple is not yet ready to be rolled.

Original change's description:
> [build] Omit the vendor component in Fuchsia target triple
> 
> This is no longer used by rustc.
> 
> Change-Id: I8b4a8d01be9195ca3f17833a69f9f3a1bb69fc61

TBR=phosek@google.com,mcgrathr@google.com,cramertj@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: Iba8d25743345451b9b954e62e748f4ee8c6d1683
diff --git a/rustc_deps/BUILD.gn b/rustc_deps/BUILD.gn
index 3c809cf..bf45141 100644
--- a/rustc_deps/BUILD.gn
+++ b/rustc_deps/BUILD.gn
@@ -81,9 +81,9 @@
     args += ["--sysroot", rebase_path("$root_build_dir/sdks/zircon_sysroot/arch/$target_cpu/sysroot") ]
     assert(current_cpu == "x64" || current_cpu == "arm64")
     if (current_cpu == "x64") {
-      target_triple = "x86_64-fuchsia"
+      target_triple = "x86_64-unknown-fuchsia"
     } else if (current_cpu == "arm64") {
-      target_triple = "aarch64-fuchsia"
+      target_triple = "aarch64-unknown-fuchsia"
     }
   }