Revert "Revert "ART: Use bionic TLS slot for thread-self""

This reverts commit 50832b6804d15ea5ddfe99a2753a09111b5bfe37.

Fix clearing the TLS on detach.

Change-Id: I75ed5ccccb4f31800e5f893055e96067a9695258
diff --git a/build/Android.common_build.mk b/build/Android.common_build.mk
index 05cfc42..3a1bd09 100644
--- a/build/Android.common_build.mk
+++ b/build/Android.common_build.mk
@@ -209,6 +209,11 @@
   external/vixl/src \
   external/zlib \
 
+# We optimize Thread::Current() with a direct TLS access. This requires access to a private
+# Bionic header.
+# Note: technically we only need this on device, but this avoids the duplication of the includes.
+ART_C_INCLUDES += bionic/libc/private
+
 # Base set of cflags used by all things ART.
 art_cflags := \
   -fno-rtti \