Revert "Clean up the AVX2 activation in the gni file"

This reverts commit 3f7298d644510773781fe52991f029410a0740d1.

Reason for revert: Speculative revert to investigate failures

Original change's description:
> Clean up the AVX2 activation in the gni file
>
> Bug: webrtc:11663
> Change-Id: Ib85ee67c72175270ddf212c7f1a09115906f0c4c
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/185480
> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
> Commit-Queue: Per Åhgren <peah@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#32202}

TBR=mbonadei@webrtc.org,peah@webrtc.org

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

Bug: webrtc:11663
Change-Id: I1d7f53237cb41750e41cd031b9dc1829f5e1646e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/186080
Reviewed-by: Per Åhgren <peah@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32253}

webrtc.gni

@@ -243,7 +243,11 @@ declare_args() {
 
   # Set this to true to enable the avx2 support in webrtc.
   # TODO: Make sure that AVX2 works also for non-clang compilers.
-  rtc_enable_avx2 = is_clang
+  if (is_clang == true) {
+    rtc_enable_avx2 = true
+  } else {
+    rtc_enable_avx2 = false
+  }
 
   # Include tests in standalone checkout.
   rtc_include_tests = !build_with_chromium && !build_with_mozilla