For i?86 negative offsets to %fs segment accesses cause a hypervisor
trap for Xen.  Avoid this by making accesses indirect.

???  Note that similar to the behavior on SLE11 this only affects
the compiler built on %ix86, not that on x86_64, even with -m32.

Index: gcc/config/i386/linux.h
===================================================================
--- gcc/config/i386/linux.h.orig	2015-12-17 15:07:37.785650062 +0100
+++ gcc/config/i386/linux.h	2015-12-17 15:08:06.393983290 +0100
@@ -24,3 +24,9 @@ along with GCC; see the file COPYING3.
 
 #undef MUSL_DYNAMIC_LINKER
 #define MUSL_DYNAMIC_LINKER "/lib/ld-musl-i386.so.1"
+
+/* This slows down Xen, so take a very small general performance hit
+   for not accessing the %fs segment with negative offsets by making
+   GCC not emit direct accesses to %fs at all.  */
+#undef TARGET_TLS_DIRECT_SEG_REFS_DEFAULT
+#define TARGET_TLS_DIRECT_SEG_REFS_DEFAULT 0