Add gcc8 package (#3127)

Remove gcc package

Update gcc7 from 7.3.0-3 to 7.4.0

Change dependency to gcc8 in buildessential

Update install.sh to use gcc8

Replace xz package with xzutils

Update postinstall message in install.sh

Add pre-built binaries for gcc7 and gcc8

packages/gcc.rb

@@ -1,27 +0,0 @@
-require 'package'
-
-class Gcc < Package
-  description 'The GNU Compiler Collection includes front ends for C, C++, Objective-C, Fortran, Ada, and Go.'
-  homepage 'https://www.gnu.org/software/gcc/'
-  version '4.9.4-1'
-
-  binary_url ({
-    aarch64: 'https://dl.bintray.com/chromebrew/chromebrew-cross/gcc-4.9.4-1-chromeos-armv7l.tar.xz',
-     armv7l: 'https://dl.bintray.com/chromebrew/chromebrew-cross/gcc-4.9.4-1-chromeos-armv7l.tar.xz',
-       i686: 'https://dl.bintray.com/chromebrew/chromebrew-cross/gcc-4.9.4-1-chromeos-i686.tar.xz',
-     x86_64: 'https://dl.bintray.com/chromebrew/chromebrew-cross/gcc-4.9.4-1-chromeos-x86_64.tar.xz',
-  })
-  binary_sha256 ({
-    aarch64: 'ecb047a9f52ea6b24313d4329b00236b6880e12df684e52e34e9664127d447c6',
-     armv7l: 'ecb047a9f52ea6b24313d4329b00236b6880e12df684e52e34e9664127d447c6',
-       i686: '9a8b20b58a564c9557c74b2b2769ddc68c110211588356b71c533aac6d54431b',
-     x86_64: '1ae1f58ca4b9fee3b0f68856e5d49b2feeb90e259e8d885f339c9aa8091f27c8',
-  })
-
-  depends_on 'binutils'
-  depends_on 'gmp'
-  depends_on 'mpfr'
-  depends_on 'mpc'
-  depends_on 'isl'
-  depends_on 'glibc'
-end