Add GNU/Linux support (not yet makefile).

chopstx-gnu-linux.h

@@ -0,0 +1,9 @@
+/*
+ * The thread context: specific to GNU/Linux.
+ *
+ * We use the type ucontext_t, which includes all registers;
+ * Note that signal mask is also included in ucontext_t.
+ *
+ */
+
+typedef ucontext_t tcontext_t;