gnu: Switch to GCC 4.8 as the default compiler.

* build-aux/download.scm (file-name->uri): Update URL to /20131110.
* gnu-system.am (nodist_bootstrap_x86_64_linux_DATA,
  nodist_bootstrap_i686_linux_DATA): Change base name to
  'guile-2.0.9.tar.xz'.
  (gnu/packages/bootstrap/x86_64-linux/guile-2.0.9.tar.xz,
  gnu/packages/bootstrap/i686-linux/guile-2.0.9.tar.xz): New targets.
  (gnu/packages/bootstrap/x86_64-linux/guile-2.0.7.tar.xz,
  gnu/packages/bootstrap/i686-linux/guile-2.0.7.tar.xz): Remove.
  (gnu/packages/bootstrap/mips64el-linux/guile-2.0.9.tar.xz): Update
  hash.
* gnu/packages/base.scm (gcc-boot0): Base upon GCC-4.8.  Enable C++, and
  add a few --disable flags.  Remove MPFR workaround.
  (cross-gcc-wrapper): Base upon GCC-4.8.  Wrap both 'gcc' and 'g++'.
  (libstdc++): New variable.
  (gcc-final): Add it as an input; pass corresponding -I and -L flags.
* gnu/packages/bootstrap.scm (%bootstrap-guile): Always use
  "guile-2.0.9.tar.xz.
  (%bootstrap-coreutils&co): Update hashes.
  (%bootstrap-binutils): Update URL and hashes.
  (%bootstrap-glibc): Likewise.
  (%bootstrap-gcc): Likewise.
* gnu/packages/gcc.scm (gcc-4.7): Add LDFLAGS.
  Add --disable-libstdcxx-pch.
  Modify the GNU_USER*_SPEC macros from gnu-user*.h instead of the raw
  _SPEC macros.