glibc: Refactor startfiles/headers into do_libc_backend()

Refactor the contents of 'do_libc_start_files()' and 'do_libc()' into a
parameterized 'do_libc_backend()'. 'do_libc_start_files()' and 'do_libc()'
call 'do_libc_backend()' with either 'libc_mode=startfiles' or
'libc_mode=final' (respectively) so that the startfiles/headers and
the final libc builds are configured and built with the same options.

One example of where this is needed is when building a mips toolchain.
Previously, if you were building an n32 toolchain, you wouldn't have
noticed an issue, because if '-mabi' is not in CFLAGS, n32 is the
default:

http://sourceware.org/git/?p=glibc-ports.git;a=blob;f=sysdeps/mips/preconfigure;hb=HEAD

But when trying to build an o32 or n64 toolchain the build would
have failed. This is because (e)glibc expects "-mabi={o32,n32,n64}" to be
in CFLAGS, but was not previously provided in 'do_libc_start_files()'.
The build failure would happen in the shared-core gcc when it tries to
configure an n64 or o32 gcc with an n32 libc.

A simpler solution would have been to just add TARGET_CFLAGS to configure
in 'do_libc_start_files()', but this way makes configure and make
consistent for both steps.

Signed-off-by: Bryan Hundven <bryanhundven@gmail.com>

 # To be sourced

 _ct-ng () {

     local cur prev samples show_samples actions steps start_steps stop_steps ct_ng_opts vars

     COMPREPLY=()

     cur=$(_get_cword)

     prev=${COMP_WORDS[COMP_CWORD-1]}

     samples=$( "${COMP_WORDS[0]}" list-samples-short 2>/dev/null )

     show_samples=$(echo "${samples}" |sed -r -e 's/(^| )/\1show-/g;')

     build_samples=$(echo "${samples}" |sed -r -e 's/(^| )/\1build-/g;')

     steps=$(${COMP_WORDS[0]} list-steps 2>/dev/null |awk '$1 == "-" { print $2; }')

     start_steps=$(echo "${steps}" |sed -r -e 's/($| )/\1+/;')

     stop_steps=$(echo "${steps}" |sed -r -e 's/(^| )/+\1/;')

     actions='help menuconfig oldconfig saveconfig extractconfig

              build build. build-all build-all.

              wiki-samples list-samples list-samples-short list-steps

              show-tuple show-all show-config

              clean distclean updatetools

              tarball version'

     vars="RESTART= STOP= PREFIX= V="

     ct_ng_opts="${samples} ${show_samples} ${build_samples}

                 ${steps} ${start_steps} ${stop_steps}

                 ${actions} ${vars}"

     COMPREPLY=($(compgen -W "${ct_ng_opts}" -- "${cur}"))

     return 0

 }

 complete -F _ct-ng ct-ng