rustc: Handle some libstd symbole exports better

Right now symbol exports, particularly in a cdylib, are handled by
assuming that `pub extern` combined with `#[no_mangle]` means "export
this". This isn't actually what we want for some symbols that the
standard library uses to implement itself, for example symbols related
to allocation. Additionally other special symbols like
`rust_eh_personallity` have no need to be exported from cdylib crate
types (only needed in dylib crate types).

This commit updates how rustc handles these special symbols by adding to
the hardcoded logic of symbols like `rust_eh_personallity` but also
adding a new attribute, `#[rustc_std_internal_symbol]`, which forces the
export level to be considered the same as all other Rust functions
instead of looking like a C function.

The eventual goal here is to prevent functions like `__rdl_alloc` from
showing up as part of a Rust cdylib as it's just an internal
implementation detail. This then further allows such symbols to get gc'd
by the linker when creating a cdylib.

src/test/run-make/cdylib-fewer-symbols/Makefile

@@ -0,0 +1,17 @@
+# Test that allocator-related symbols don't show up as exported from a cdylib as
+# they're internal to Rust and not part of the public ABI.
+
+-include ../tools.mk
+
+ifdef IS_MSVC
+all:
+	true
+else
+all:
+	$(RUSTC) foo.rs
+	nm -g "$(call DYLIB,foo)"
+	nm -g "$(call DYLIB,foo)" | grep -vq __rdl_
+	nm -g "$(call DYLIB,foo)" | grep -vq __rde_
+	nm -g "$(call DYLIB,foo)" | grep -vq __rg_
+	nm -g "$(call DYLIB,foo)" | grep -vq __rust_
+endif

src/test/run-make/cdylib-fewer-symbols/foo.rs

@@ -0,0 +1,16 @@
+// Copyright 2017 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_type = "cdylib"]
+
+#[no_mangle]
+pub extern fn foo() -> u32 {
+    3
+}