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
It's useful to capture the platform and platform version with the image map.
Also, display both the platform and architecture information when generating
a crash log.
rdar://124913332
We want to be able to efficiently serialise lists of images, and to do so
it makes most sense to create a separate `ImageMap` type. This also provides
a useful place to put methods to e.g. find an image by address or by build
ID.
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
