linux-user: Enable NPTL for m68k

For m68k, per-thread data is a purely kernel construct with no
CPU level support. Implement it via a field in the TaskState structure,
used by cpu_set_tls() and the set_thread_area/get_thread_area
syscalls. This allows us to enable compilation with NPTL.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by:  Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
diff --git a/linux-user/qemu.h b/linux-user/qemu.h
index 8c420da..1ff0fa8 100644
--- a/linux-user/qemu.h
+++ b/linux-user/qemu.h
@@ -121,6 +121,7 @@
 #endif
 #ifdef TARGET_M68K
     int sim_syscalls;
+    abi_ulong tp_value;
 #endif
 #if defined(TARGET_ARM) || defined(TARGET_M68K) || defined(TARGET_UNICORE32)
     /* Extra fields for semihosted binaries.  */