hastily port so we don't fail to build

src/rt/arch/x86_64/morestack.S

@@ -0,0 +1,52 @@
+    .text
+
+// __morestack
+//
+// LLVM generates a call to this to allocate more stack space in a functiono
+// prolog when we run out.
+
+#if defined(__APPLE__) || defined(_WIN32)
+#define RUST_NEW_STACK      _rust_new_stack
+#define RUST_DEL_STACK      _rust_del_stack
+#else
+#define RUST_NEW_STACK      rust_new_stack
+#define RUST_DEL_STACK      rust_del_stack
+#endif
+
+        // Naturally, nobody can agree as to
+        // which arguments should go in which
+        // registers:
+#if defined(_WIN32)
+#  define ARG0 %rcx
+#  define ARG1 %rdx
+#  define ARG2 %r8
+#else
+#  define ARG0 %rdi
+#  define ARG1 %rsi
+#  define ARG2 %rdx
+#endif
+
+.globl RUST_NEW_STACK
+.globl RUST_DEL_STACK
+
+.globl __morestack
+
+__morestack:
+        // Hastily and probably incorrectly ported from i386 version.
+        // Actually this calling convention doens't make so much sense
+        // for x86_64...
+        mov %rcx, ARG0      // param 0: amount of space needed
+        mov %rdx, ARG2      // param 2: size of arguments
+        lea 8(%rsp),ARG1
+        call RUST_NEW_STACK
+
+        mov (%rsp),%rdx        // Grab the return pointer.
+        inc %rdx               // Skip past the `ret`.
+        mov %rax,%rsp          // Switch to the new stack.
+        call *%rdx             // Enter the new function.
+
+        // Now the function that called us has returned, so we need to delete the
+        // old stack space.
+        call RUST_DEL_STACK
+        mov %rax,%rsp          // Switch back to the old stack.
+        ret