swift: refactor phases and patch documentation

* 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

pkgs/development/compilers/swift/purity.patch

@@ -1,14 +1,5 @@
-From 4add81bba40dcec62c4ea4481be8e35ac53e89d8 Mon Sep 17 00:00:00 2001
-From: Will Dietz <w@wdtz.org>
-Date: Thu, 18 May 2017 11:56:12 -0500
-Subject: [PATCH] "purity" patch for 5.0
+"purity" patch for 5.0
 
----
- lib/Driver/ToolChains/Gnu.cpp | 7 -------
- 1 file changed, 7 deletions(-)
-
-diff --git a/lib/Driver/ToolChains/Gnu.cpp b/lib/Driver/ToolChains/Gnu.cpp
-index fe3c0191bb..c6a482bece 100644
 --- a/lib/Driver/ToolChains/Gnu.cpp
 +++ b/lib/Driver/ToolChains/Gnu.cpp
 @@ -380,13 +380,6 @@ void tools::gnutools::Linker::ConstructJob(Compilation &C, const JobAction &JA,
@@ -25,6 +16,3 @@ index fe3c0191bb..c6a482bece 100644
    }
  
    CmdArgs.push_back("-o");
--- 
-2.11.0
-