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/0001-build-presets-linux-don-t-require-using-Ninja.patch

@@ -1,14 +1,5 @@
-From 1fc49285c7a198de14005803dfde64bda17f4120 Mon Sep 17 00:00:00 2001
-From: Will Dietz <w@wdtz.org>
-Date: Tue, 28 Mar 2017 15:01:16 -0500
-Subject: [PATCH 1/4] build-presets: (linux) don't require using Ninja
+Don't build Ninja, we use our own.
 
----
- utils/build-presets.ini | 2 +-
- 1 file changed, 1 insertion(+), 1 deletion(-)
-
-diff --git a/utils/build-presets.ini b/utils/build-presets.ini
-index 7ee57ad2df..e6b0af3581 100644
 --- a/utils/build-presets.ini
 +++ b/utils/build-presets.ini
 @@ -721,7 +721,7 @@ swiftpm
@@ -20,6 +11,3 @@ index 7ee57ad2df..e6b0af3581 100644
  install-swift
  install-lldb
  install-llbuild
--- 
-2.12.2
-