Add weak `std::terminate()` implementation

This allows calling it from other places later. The default
implementation calls `abort()`, but making it weak allows user code to
override this function (either directly, or by including a library like
uclibc++ that implements `std::set_terminate()`).

Note that this does not add a declaration for this function, since the
standard dictates this to be in `<exception>`, but we cannot
meaningfully or completely implement that header, so better leave it to
be overridden by e.g. libraries like uclibc++.

cores/arduino/abi.cpp

@@ -21,6 +21,12 @@
 extern "C" void __cxa_pure_virtual(void) __attribute__ ((__noreturn__));
 extern "C" void __cxa_deleted_virtual(void) __attribute__ ((__noreturn__));
 
+namespace std {
+  [[gnu::weak, noreturn]] void terminate() {
+    abort();
+  }
+}
+
 void __cxa_pure_virtual(void) {
   // We might want to write some diagnostics to uart in this case
   //std::terminate();