build gcc on X86

refer to https://gcc.gnu.org/install/

Download GCC source:

1. svn checkout svn://gcc.gnu.org/svn/gcc/trunk gcc-trunk

2.enter gcc-trunk and run download_prerequisties:

gcc-trunk>contrib/download_prerequisites

Configure GCC:

% mkdir objdir
% cd objdir
% srcdir/configure

Build GCC:

>cd objdir

objdir>make

Install GCC:

objdir>make install

xxxx@ubuntu:~$ /usr/local/bin/gcc --version
gcc (GCC) 9.0.0 20181024 (experimental)
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.