relax host system requirements

A requirement on Glibc is not needed at all.  It's enough once
$LFS_TGT-* is runnable.  A test on Alpine (using musl as libc) has
practically proved this.

We'd raised binutils and GCC requirements mostly for Glibc.  But now
Glibc is cross compiled by our cross toolchain with latest GCC and
binutils release, the host tools really does not matter.  In the Glibc
building process only two .c files are build with BUILD_CC (the C
compiler from the host), and they are highly conservative (mostly
unchanged for years).

Binutils does not have too much requirement on host GCC & Binutils:
there is even a Binutils commit in this week fixing a build failure with
GCC-4.2!

So the most strict limitation comes from GCC.  GCC requires host GCC to
support ISO C++ 11 so GCC >= 4.8 is needed.  And both GCC-4.8 and latest
GCC-11.2 claims a requirement for Binutils-2.12 (for x86_64) or 2.13.1
(for 32-bit x86), so we make minimal Binutils version 2.13.1.

And, host bzip2 is never used now: the only .tar.bz2 files are elfutils
and python docs.  They are not decompressed before entering chroot.

chapter05/gcc-pass1.xml

@@ -107,11 +107,10 @@ cd       build</userinput></screen>
         <term><parameter>--with-glibc-version=&glibc-version;</parameter></term>
         <listitem>
           <para>This option specifies the version of glibc which will be
-          used on the target. It is not relevant to the minimum glibc
-          requirement specified in the
-          <xref linkend="ch-partitioning-hostreqs"/> because everything
-          compiled by pass1 gcc will run in the chroot environment,
-          which is isolated from glibc of the host distro.</para>
+          used on the target. It is not relevant to the libc of the host
+          distro because everything compiled by pass1 gcc will run in the
+          chroot environment, which is isolated from libc of the host
+          distro.</para>
         </listitem>
       </varlistentry>