Use LLVM's bundled RPMalloc

Using LLVM's bundled RPMalloc lets us get rid of our own download code
and also works around a clang test failure that is resolved by updating
RPMalloc.

Bug: 351660506,355302234
Fixed: 351660506,355302234
Change-Id: Ia022f609907b78bc2de98248202e4bcfdb13e8c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5738937
Reviewed-by: Alan Zhao <ayzhao@google.com>
Commit-Queue: Arthur Eubanks <aeubanks@google.com>
Auto-Submit: Alan Zhao <ayzhao@google.com>
Reviewed-by: Arthur Eubanks <aeubanks@google.com>
Cr-Commit-Position: refs/heads/main@{#1335589}
NOKEYCHECK=True
GitOrigin-RevId: 26be252f7cc9c210806bcfc02874cfcae5259c18

Author: aeubanks

update_rust.py

@@ -36,7 +36,7 @@
 # this back to its previous value _AND_ set `OVERRIDE_CLANG_REVISION` below
 # to the `CLANG_REVISION` that was in place before the roll.
 RUST_REVISION = '3cf924b934322fd7b514600a7dc84fc517515346'
-RUST_SUB_REVISION = 3
+RUST_SUB_REVISION = 4
 
 # If not None, this overrides the `CLANG_REVISION` in
 # //tools/clang/scripts/update.py in order to download a Rust toolchain that