libc/newlib: build newlib in the proper step

Currently, newlib is built in the start_file step, which is wrong, but was
needed when the baremetal integration was... well, 'unfinished'.

Now that we build the baremetal compiler from the final cc step, and a
proper core gcc in pass-1 and pass-2, we can move the newlib build to the
step do_libc, where it belongs.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 --- glibc-2_9.orig/nptl/init.c	2007-10-15 22:25:25.000000000 +0200

 +++ glibc-2_9/nptl/init.c	2010-08-18 10:46:06.109098646 +0200

 @@ -236,7 +236,7 @@

  /* This can be set by the debugger before initialization is complete.  */

 -static bool __nptl_initial_report_events;

 +static bool __nptl_initial_report_events __attribute_used__;

  void

  __pthread_initialize_minimal_internal (void)