Include cxa_thread_atexit on linux targets.

Per @thomasanderson, the current minimum requirement is GLIBC 2.19.

Bug: b/145962051, b/147656572
Change-Id: I1625753c6b0a2332015b78394740f0dba5a01519
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2003254Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Felipe Salazar <felipesalazar@chromium.org>
Auto-Submit: Felipe Salazar <felipesalazar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732222}

buildtools/third_party/libc++abi/BUILD.gn

@@ -54,9 +54,7 @@ source_set("libc++abi") {
     sources += [ "trunk/src/cxa_demangle.cpp" ]
   }
 
-  # This file should really be included on linux as well, but that
-  # would introduce an unwanted glibc 2.18 dependency.
-  if (is_fuchsia || (is_posix && !is_mac && !is_ios && !is_linux)) {
+  if (is_fuchsia || (is_posix && !is_mac && !is_ios)) {
     sources += [ "trunk/src/cxa_thread_atexit.cpp" ]
   }