This comes with a number of fixes to be compatible with upstream LLVM:

* Previously all monomorphizations of "mem::size_of()" would receive the same
  symbol. In the past LLVM would silently rename duplicated symbols, but it
  appears to now be dropping the duplicate symbols and functions now. The symbol
  names of monomorphized functions are now no longer solely based on the type of
  the function, but rather the type and the unique hash for the
  monomorphization.

* Split stacks are no longer a global feature controlled by a flag in LLVM.
  Instead, they are opt-in on a per-function basis through a function attribute.
  The rust #[no_split_stack] attribute will disable this, otherwise all
  functions have #[split_stack] attached to them.

* The compare and swap instruction now takes two atomic orderings, one for the
  successful case and one for the failure case. LLVM internally has an
  implementation of calculating the appropriate failure ordering given a
  particular success ordering (previously only a success ordering was
  specified), and I copied that into the intrinsic translation so the failure
  ordering isn't supplied on a source level for now.

* Minor tweaks to LLVM's API in terms of debuginfo, naming, c++11 conventions,
  etc.

When clang is enabled, also pass through --enable-libcpp to LLVM's configure
command line to help it pick up the most recent c++ runtime library. This also
changes the mklldeps.py script to pick up on whether LLVM was linked against
stdc++ or c++ based on the --cxxflags that llvm-config prints.

In an ongoing attempt to update LLVM, the bots need to update their C compilers
to something that supports c++11 (LLVM recently switched). The OSX bots are
running Lion (10.7), which only supports up to gcc 4.2 and clang 3.2. Apparently
the libstdc++ is too old (even on the most updated command line tools) for LLVM,
but using libc++ instead appears to work just fine.

OSX often has a more recent version of clang than it does for GCC. When an older
version of gcc is detected on OSX, the --enable-clang flag is implicitly
enabled.

Older version of LLVM did not have this flag, so we need to fall back to our
previous library detection when using older versions of LLVM.

The goal of the snapshot bots is to produce binaries which can run in as many
locations as possible. Currently we build on Centos 6 for this reason, but with
LLVM's update to C++11, this reduces the number of platforms that we could
possibly run on.

This adds a --enable-llvm-static-stdcpp option to the ./configure script for
Rust which will enable building a librustc with a static dependence on
libstdc++. This normally isn't necessary, but this option can be used on the
snapshot builders in order to continue to make binaries which should be able to
run in as many locations as possible.

If a linker finds both a static and a dynamic version of the same library, then
the linker often chooses the dynamic version. This is surprising when a native
library is specified as being "static" in rust source. This modifies the linker
command line to obey the hints given in rust source files and instructing the
linker to prefer a particular version of a found library.

Unfortunately, this patch has no effect on osx because the linker supports
no such hint, and it also has no effect on windows because the linker apparently
just ignores it. For now this is predominately used to enable the previous patch
of linking to libstdc++ statically, but more support would need to be added for
this in the future if we wanted to officially support it.

cc rust-lang#12557 (doesn't close because it doesn't support OSX and windows)

This is a consequence of rust-lang#13611 and our bots running a "fairly old" gdb which
doesn't understand the newer versions of dwarf.