Add a method of manually specifying the crate map

Apparently weak linkage and dlopen aren't quite working out for applications
like servo on android. There appears to be a bug or two in how android loads
dynamic libraries and for some reason libservo.so isn't being found.

As a temporary solution, add an extern "C" function to libstd which can be
called if you have a handle to the crate map manually. When crawling the crate
map, we then check this manual symbol before falling back to the old solutions.

cc #11731

src/test/run-make/c-set-crate-map-manually/Makefile

@@ -0,0 +1,7 @@
+-include ../tools.mk
+
+all:
+	$(RUSTC) lib.rs -C gen-crate-map
+	ln -nsf $(call DYLIB,boot-*) $(call DYLIB,boot)
+	$(CC) main.c -o $(call RUN,main) -lboot -Wl,-rpath,$(TMPDIR)
+	RUST_LOG=boot $(call RUN,main)

src/test/run-make/c-set-crate-map-manually/lib.rs

@@ -0,0 +1,30 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[crate_id="boot#0.1"];
+#[crate_type="dylib"];
+#[no_uv];
+
+extern crate rustuv;
+extern crate green;
+
+use std::rt::crate_map::{CrateMap, rust_set_crate_map};
+
+// pull in this symbol from libstd into this crate (for convenience)
+#[no_mangle]
+pub static set_crate_map: extern "C" fn(*CrateMap<'static>) = rust_set_crate_map;
+
+#[no_mangle] // this needs to get called from C
+pub extern "C" fn foo(argc: int, argv: **u8) -> int {
+    green::start(argc, argv, proc() {
+        if log_enabled!(std::logging::DEBUG) { return }
+        fail!()
+    })
+}

src/test/run-make/c-set-crate-map-manually/main.c

@@ -0,0 +1,20 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// this is the rust entry point that we're going to call.
+int foo(int argc, char *argv[]);
+
+extern void (*set_crate_map)(void *map);
+extern int _rust_crate_map_toplevel;
+
+int main(int argc, char *argv[]) {
+  set_crate_map(&_rust_crate_map_toplevel);
+  return foo(argc, argv);
+}