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>

 diff -dur ELFkickers.orig/sstrip/Makefile ELFkickers/sstrip/Makefile

 --- ELFkickers.orig/sstrip/Makefile	2001-03-24 12:58:27.000000000 +0100

 +++ ELFkickers/sstrip/Makefile	2007-05-31 21:17:18.000000000 +0200

 @@ -1,7 +1,7 @@

  #  Makefile for sstrip

  sstrip: sstrip.c

 -	gcc -ggdb -Wall -W -o sstrip sstrip.c

 +	$(CC) -Wall -W -o sstrip sstrip.c

  clean:

  	rm -f sstrip