Auto merge of #32239 - alexcrichton:fix-cross-to-freebsd, r=brson

Auto merge of #32239 - alexcrichton:fix-cross-to-freebsd, r=brson

rustbuild: Fix cross compiling to FreeBSD

This commit fixes our support for cross compiling a compiler to run on FreeBSD.
Over the weekend I managed to get a cross compiler from Linux to FreeBSD [1]
which I hope to soon use to start producing FreeBSD nightly compilers. With the
`make dist` support added in #32237 we should be able to produce standard
rustc/rust-std packages for FreeBSD through a new slave with this cross compiler.

Currently, however, we don't "Just Work" when cross compiling FreeBSD and a
number of changes were required (part of this PR). They include:

* A few build fixes were needed in LLVM. Our own branch has been rebased on the
  actual 3.8 release and I applied one extra commit [2] which contains two fixes:

  1. The LLVM CMake build system passes the `-Wl,-z,defs` flag on many
     platforms, but *not* when `CMAKE_SYSTEM_NAME` is "FreeBSD". Unfortunately
     this doesn't take into account when we're cross compiling, and as predicted
     the build will fail if `-Wl,-z,defs` is passed (see [3] for more info). To
     fix this we test `TARGET_TRIPLE` instead of the `CMAKE_SYSTEM_NAME` which
     is what we're compiling for which fixes the problem.
  2. The `PATH_MAX` constant is apparently defined in a different location than
     many other Unix systems, so a file which required this just needed some
     help to keep compiling.

* Support for compiling compiler-rt with CMake has been added to rustbuild. It
  looks like it just emulates Linux in what it compiles as it didn't seem to
  naturally produce anything else... At least the architecture is right, so
  seems good for now at least!

[1]: https://github.com/alexcrichton/port-of-rust/blob/master/prebuilt/freebsd/Dockerfile
[2]: https://github.com/rust-lang/llvm/commit/be89e4b5
[3]: https://bugs.webkit.org/show_bug.cgi?id=138420