cflags: Upgrade -mavx -> -mavx2

Some new use cases in Trace Processor require
AVX2. It seems supported by most CPUs since 2013
according to
https://en.wikipedia.org/wiki/Advanced_Vector_Extensions#CPUs_with_AVX2

Bug: 205302474
Change-Id: I8d6a43d6c8544b454b7e1b8220baced06aef8f67
diff --git a/CHANGELOG b/CHANGELOG
index ab2f7bf..db4ed93 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -13,6 +13,8 @@
       an ISO8601 string.
     * Changed 'trace_to_text' to be named 'traceconv'. The source
       location also moved from 'tools/trace_to_text' to 'src/traceconv'.
+    * Changed compiler flags, added -mavx2. The previous patch in v22.0 was
+      supposed to add AVX2 support but added only AVX.
   UI:
     *
   SDK: