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/patches/0004-build-presets-linux-plumb-extra-cmake-options.patch

@@ -1,14 +1,5 @@
-From 4a46b12f580d0a9779937d07c4f1fd347570c4ef Mon Sep 17 00:00:00 2001
-From: Will Dietz <w@wdtz.org>
-Date: Tue, 28 Mar 2017 15:02:37 -0500
-Subject: [PATCH 4/4] build-presets: (linux) plumb extra-cmake-options
+plumb extra-cmake-options
 
----
- utils/build-presets.ini | 2 ++
- 1 file changed, 2 insertions(+)
-
-diff --git a/utils/build-presets.ini b/utils/build-presets.ini
-index 1739e91dc2..0608fed9c1 100644
 --- a/utils/build-presets.ini
 +++ b/utils/build-presets.ini
 @@ -743,6 +743,8 @@ install-destdir=%(install_destdir)s
@@ -20,6 +11,3 @@ index 1739e91dc2..0608fed9c1 100644
  [preset: buildbot_linux]
  mixin-preset=mixin_linux_installation
  build-subdir=buildbot_linux
--- 
-2.12.2
-