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>

 # Wrapper to build the test suite facilities

 #

 # Current assumption: test suites are independent of each other

 #                     - no order handling required.

 # List all test suite facilities, and parse their scripts

 CT_TEST_SUITE_FACILITY_LIST=

 for f in "${CT_LIB_DIR}/scripts/build/test_suite/"*.sh; do

     _f="$(basename "${f}" .sh)"

     __f="CT_TEST_SUITE_${_f}"

     __f=`echo ${__f} | tr "[:lower:]" "[:upper:]"`

     if [ "${!__f}" = "y" ]; then

         CT_DoLog DEBUG "Enabling test suite '${_f}'"

         . "${f}"

         CT_TEST_SUITE_FACILITY_LIST="${CT_TEST_SUITE_FACILITY_LIST} ${_f}"

     else

         CT_DoLog DEBUG "Disabling test suite '${_f}'"

     fi

 done

 # Download the test suite facilities

 do_test_suite_get() {

     for f in ${CT_TEST_SUITE_FACILITY_LIST}; do

         do_test_suite_${f}_get

     done

 }

 # Extract and patch the test suite facilities

 do_test_suite_extract() {

     for f in ${CT_TEST_SUITE_FACILITY_LIST}; do

         do_test_suite_${f}_extract

     done

 }

 # Build the test suite facilities

 do_test_suite() {

     for f in ${CT_TEST_SUITE_FACILITY_LIST}; do

         do_test_suite_${f}_build

     done

 }