Implement wasm trap handlers. (#27)

* Implement wasm trap handlers.

This adds signal handlers based on SpiderMonkey's signal-handler code.
The functionality for looking up the trap code and wasm bytecode offset
isn't yet implemented, but this is a start.

I considered rewriting this code in Rust, but decided against it for now
as C++ allows us to talk to the relevant OS APIs more directly.

Fixes #15.

* Compile with -std=c++11.

* Refactor InstallState initialization.

* Compile with -fPIC.

* Factor out the code for calling a wasm function with a given index.

* Fix unclear wording in a comment.

lib/execute/signalhandlers/SignalHandlers.h

@@ -0,0 +1,55 @@
+#ifndef signal_handlers_h
+#define signal_handlers_h
+
+#include <stdint.h>
+#include <setjmp.h>
+#ifndef __cplusplus
+#include <stdbool.h>
+#endif
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+struct CodeSegment;
+
+// Record the Trap code and wasm bytecode offset in TLS somewhere
+void RecordTrap(const uint8_t* pc, const struct CodeSegment* codeSegment);
+
+// Initiate an unwind.
+void Unwind(void);
+
+// Return the CodeSegment containing the given pc, if any exist in the process.
+// This method does not take a lock.
+const struct CodeSegment*
+LookupCodeSegment(const void* pc);
+
+// Trap initialization state.
+struct TrapContext {
+    bool triedToInstallSignalHandlers;
+    bool haveSignalHandlers;
+};
+
+// This function performs the low-overhead signal handler initialization that we
+// want to do eagerly to ensure a more-deterministic global process state. This
+// is especially relevant for signal handlers since handler ordering depends on
+// installation order: the wasm signal handler must run *before* the other crash
+// handlers and since POSIX signal handlers work LIFO, this function needs to be
+// called at the end of the startup process, after other handlers have been
+// installed. This function can thus be called multiple times, having no effect
+// after the first call.
+bool
+EnsureEagerSignalHandlers(void);
+
+// Assuming EnsureEagerProcessSignalHandlers() has already been called,
+// this function performs the full installation of signal handlers which must
+// be performed per-thread. This operation may incur some overhead and
+// so should be done only when needed to use wasm.
+bool
+EnsureDarwinMachPorts(struct TrapContext* cx);
+
+#ifdef __cplusplus
+} // extern "C"
+#endif
+
+#endif // signal_handlers_h