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]: be89e4b5
[3]: https://bugs.webkit.org/show_bug.cgi?id=138420
483fc715c6