Commit for CMake and build scripts to recognize OpenBSD. To keep this
commit relatively short, this just deals with the rather simple and
uncontroversial changes to the build system.
Note that OpenBSD calls "x86_64" as "amd64", Since the Swift stdlib will
be put in a subdirectory named after ARCH, to ensure the standard
library is properly found later, we use the native architecture name for
OpenBSD in the build system rather than trying to deal with the
difference the other way around.
There are situations where you want to build against a libc that is out
of tree or that is not the system libc (Or for cross build scenarios).
This is a change for passing the -sdk and include paths for things like
this.
Magic symbols of the form $ld$install_name$os9.0$@rpath/libswiftCore.dylib tell the linker to use that install name when targeting that OS version. Use these symbols to specify an @rpath install name for all back-deployment libraries when targeting watchOS 2.0-5.1, iOS 7.0-12.1, and macOS 10.9-10.14.
rdar://problem/45027809
This is in preparation to make the code here more target agnostic for
porting to the Windows threading primitives. This is used pretty
extensively in the tests, so disabling tests would lose a chunk of
coverage.
