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.
