Drop unused variable HAIKU_GCC_GLUE_CODE_<target_arch>
Use crtbeginS.o and crtendS.o everywhere.

* Gcc is now using __cxa_atexit, so we need to use the crtbegin
  and crtend implementations that are meant to be used for shared
  libraries. This avoids crashes of servers that load add-ons
  (Media-Addon-Server and Print-Server) when shutting down Haiku.
* As executable are shared on Haiku, we use crtbeginS.o and crtendS.o
  for those, too.
* To simplify, we even use crtbeginS.o and crtendS.o in the kernel,
  but there they don't currently make a difference, as the respective
  initialization and cleanup functions are not being invoked by the
  kernel.