The ELF specification says that if a section's type is set to `SHT_NULL`,
_none_ of the other fields are valid, so we shouldn't even be trying to
read the section name.
(We won't crash, because of how all of this code works, but we might
read an incorrect name.)
rdar://154837649
Musl's `clone()` wrapper returns `EINVAL` if you try to use `CLONE_THREAD`,
which seems a bit wrong (certainly it is in this particular application,
since we *really* don't care whether the thread is a valid C library
thread at this point).
Also properly support ELF images that are built with a base address other
than zero (this typically isn't an issue for dynamically linked programs,
as they will be relocated at runtime anyway, but for statically linked
binaries it's very common to set the base address to a non-zero value).
rdar://154282813
When fixing the previous problem, I should have realised that this
would cascade; the `@_specialize` annotations refer to the cached
versions of the types, and those are now turned off except on macOS
and Linux because they rely on the uncached versions, which are only
available on macOS and Linux.
rdar://144023438
Also removed a spurious `print()` that got left in by accident,
and disabled the `CodableBacktrace.swift` test on Linux since we
need Foundation and we don't have it there.
rdar://124913332
Move the backtracing code into a new Runtime module. This means renaming
the Swift Runtime's CMake target because otherwise there will be a name
clash.
rdar://124913332
