This commit turns on landing pads for MSVC by default, which means that we'll
now be running cleanups for values on the stack when an exception is thrown.
This commit "fixes" the previously seen LLVM abort by attaching the `noinline`
attribute to all generated drop glue to prevent landing pads from being inlined
into other landing pads.

The performance of MSVC is highly likely to decrease from this commit, but there
are various routes we can taken in the future if this ends up staying for quite
a while, such as generating a shim function only called from landing pads which
calls the actual drop glue, and this shim is marked noinline.

For now, however, this patch enables MSVC to successfully bootstrap itself!

This commit alters the compiler to no longer "just run link.exe" but instead
probe the system's registry to find where the linker is located. The default
library search path (normally found through LIB) is also found through the
registry. This also brings us in line with the default behavior of Clang, and
much of the logic of where to look for information is copied over from Clang as
well. Finally, this commit removes the makefile logic for updating the
environment variables for the compiler, except for stage0 where it's still
necessary.

The motivation for this change is rooted in two positions:

* Not having to set up these environment variables is much less hassle both for
  the bootstrap and for running the compiler itself. This means that the
  compiler can be run outside of VS shells and be run inside of cmd.exe or a
  MSYS shell.

* When dealing with cross compilation, there's not actually a set of environment
  variables that can be set for the compiler. This means, for example, if a
  Cargo compilation is targeting 32-bit from 64-bit you can't actually set up
  one set of environment variables. Having the compiler deal with the logic
  instead is generally much more convenient!

Just like the original article our Windows TLS support is based on predicted,
this symbol must be linked in on MSVC to pull in the necessary support for TLS
variables. This commit fixes a number of unit tests which require that TLS
destructors are run.

The MSVC compiler doesn't like empty structs, so this test won't link on MSVC,
so it's ignored.

The function is apparently just called lgamma on MSVC

This library has no shims which are actually needed on Windows now, so translate
that last easy one into Rust and then don't link it at all on Windows.

This is currently quite buggy in LLVM from what I can tell, so just disable it
entirely. This commit also adds preliminary support, however, to actually
target 32-bit MSVC by making sure the `rust_try_msvc_32.ll` file exists and
wiring up exceptions to `_except_handler3` instead of `__C_specific_handler`
(which doesn't exist on 32-bit).