Update README after changes to toolchain development process

Bug: http://b/403137505
Change-Id: I0b9393bee0a747bb89f0517fea9b28ad79aa4ead

prebuilts/clang/host/linux-x86

diff --git a/prebuilts/clang/host/linux-x86 b/prebuilts/clang/host/linux-x86
index 021cfef..70a12c4 160000
--- a/prebuilts/clang/host/linux-x86
+++ b/prebuilts/clang/host/linux-x86
@@ -1 +1 @@
-Subproject commit 021cfef0cd70d58bb71908749e492b8913512ac4
+Subproject commit 70a12c4fe88940ab2b0e6d763492dffa2f07e15a