toolchain technical note: rewrite the descrption for triplet

- Don't say "most building system", refine the dicussion for autoconf.
  Other building systems may use a variant of triplet, or use a
  completely different system designation.
- Explain why a triplet may contain 4 fields in detail. "Histroical
  reason" is not really correct because 3-field triplet is still used
  today for BSD, Fuchsia, IOS, Mac OS X (darwin), Solaris, etc.
- "machine" triplet to "system" triplet (strictly speaking, only the
  first field in the triplet is for the machine).

Why we need to say "vendor can be omitted" explicitly: we mention "gcc
-dumpmachine".  On some distros (like Ubuntu) the output has no vendor
field.  If you think this is too nasty, please remove both.

part3intro/toolchaintechnotes.xml

@@ -145,15 +145,28 @@
     <title>Implementation of Cross-Compilation for LFS</title>
 
     <note>
-      <para>Almost all the build systems use names of the form
-      cpu-vendor-kernel-os, referred to as the machine triplet. (Sometimes,
-      the vendor field is omitted.) An astute
-      reader may wonder why a <quote>triplet</quote> refers to a four component
-      name. The reason is historical: initially, three component names were enough
-      to designate a machine unambiguously, but as new machines and systems
-      proliferated, that proved insufficient. The word <quote>triplet</quote>
-      remained. A simple way to determine your machine triplet is to run
-      the <command>config.guess</command>
+      <para>All packages involved with cross compilation in the book use an
+      autoconf-based building system.  The autoconf-based building system
+      accepts system types in the form cpu-vendor-kernel-os,
+      referred to as the system triplet.  Since the vendor field is mostly
+      irrelevant, autoconf allows to omit it. An astute reader may wonder
+      why a <quote>triplet</quote> refers to a four component name. The
+      reason is the kernel field and the os field originiated from one
+      <quote>system</quote> field.  Such a three-field form is still valid
+      today for some systems, for example
+      <literal>x86_64-unknown-freebsd</literal>.  But for other systems,
+      two systems can share the same kernel but still be too different to
+      use a same triplet for them.  For example, an Android running on a
+      mobile phone is completely different from Ubuntu running on an ARM64
+      server.  Without an emulation layer, you cannot run an executable for
+      the server on the mobile phone or vice versa.  So the
+      <quote>system</quote> field is separated into kernel and os fields to
+      designate these systems unambiguously.  For our example, the Android
+      system is designated <literal>aarch64-unknown-linux-android</literal>,
+      and the Ubuntu system is designated
+      <literal>aarch64-unknown-linux-gnu</literal>.  The word
+      <quote>triplet</quote> remained. A simple way to determine your
+      system triplet is to run the <command>config.guess</command>
       script that comes with the source for many packages. Unpack the binutils
       sources and run the script: <userinput>./config.guess</userinput> and note
       the output. For example, for a 32-bit Intel processor the