Update gcc version patches (Fixes #64)

* Alpha & rs6000 architectures do not use the generic linux.h file
  therefore explicitly add the TARGET_OS_D_BUILTINS macro to
  alpha/linux.h, rs6000/linux64.h and rs6000/linux.h.

* Also remove all changes to sysv4.h headers as gcc doesn't support
  sysv4 targets anyway. This also fixes the macro redefinition
  warnings in these files.