Revert "[roll] Roll Rust toolchain"

This reverts commit 4bc48a5cbca952a8d865c1455b5c9aab07604e76.

Reason for revert: This version of Rust toolchain is broken because of ICE caused by LLVM regression as well as TLS issue affecting AArch64.

Original change's description:
> [roll] Roll Rust toolchain
> 
> Roll to revision 866a713258915e6cbb212d135f751a6a8c9e1c0a.
> 
> This changes rustc to use shorter target triples for Fuchsia, omitting
> the vendor component which also requires build configuration update.
> 
> Change-Id: Ia38f4de06f137003a3499e52166fb1f2ea3fc0aa

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

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

Change-Id: Id43dc80652ab9355cee9ace26785e3d505c1c8f3
diff --git a/fuchsia.ensure b/fuchsia.ensure
index f0ccbf0..ebb040b 100644
--- a/fuchsia.ensure
+++ b/fuchsia.ensure
@@ -45,7 +45,7 @@
 
 # Rust
 @Subdir rust
-fuchsia/rust/${platform} git_revision:866a713258915e6cbb212d135f751a6a8c9e1c0a
+fuchsia/rust/${platform} git_revision:8acec1f9d0b40dde142e6c26d7358b9ab232d2b4
 
 # Swift
 @Subdir swift