gcc: bump 6.x series to version 6.4.0

Drop the following patches:
  * the Xtensa patches 870 and 871 are upstream now
  * patch 942 was backported to GCC 6 branch

Note, that a bz2 release tarball is not provided anymore and is replaced by
a xz tarball file.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>