https://hydra.nixos.org/build/123279517
Internally used `compiler-rt` had to be fixed for `glibc-2.31`,
basically the same approach as in the LLVM fix
(7137183bbe05738246be2be0e704c1be9bf19947).
As soon as a newer `compiler-rt` is used for `swift`, this patch can be
removed again.
* simplify versioning
* uniform patch documentation
* reorder phases so they read in order
* reorder scripts for readability
* GNU toolchain dir:
handle sysroot for GNU toolchain like for C headers, so that
GCC_INSTALL_PREFIX is actually used correctly
update Swift
* new tweaks to the build process are mostly documented inline
* the swiftc wrapper needs the linker path for libicu
* some older tweaks could be removed
* remove fuzz from patches by updating line numbers
* one build fix to libdispatch has been filed upstream
* necessary tweaks and patches to get swift 4.2.1 to build
* disabled more tests because they would require additional patching
* delete empty directories from final build output
* some minor cleanup
