In the comments it's stated that these errnos can't really occur for
the implementation, but unfortunately they have been seen in some
scenarios. To aide in debugging, it'd be nice to at least have the
errno in the error message.
On all platforms except Linux, calling `withLockIfAvailable()`
recursively just returns `nil`. However, our Linux implementation
chooses to abort the process instead. We don't need this
inconsistent/destructive behaviour and can just return `nil` as we do
elsewhere.
add_swift_target_library was missing pieces for passing sources and
flags to the static SDK build. As a result, the static SDK was missing
pieces (specifically Mutex).
Also adding the Musl import to the Linux Mutex implementation.
