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>

 # elf2flt options

 if ARCH_BINFMT_FLAT

 comment "elf2flt"

 choice

     bool

     prompt "elf2flt version"

 # Don't remove next line

 # CT_INSERT_VERSION_BELOW

 config ELF2FLT_CVSHEAD

     bool

     prompt "CVS Head"

     help

       Grab the latest version of elf2flt from the CVS repository

 config ELF2FLT_CVS_SNAPSHOT

     bool

     prompt "CVS Snapshot"

 endchoice

 config ELF2FLT_CVS_SNAPSHOT_SPEC

     string

     depends on ELF2FLT_CVS_SNAPSHOT

     default ""

     prompt "CVS refspec for elf2flt"

     help

        What you enter here will be passed verbatim to the cvs checkout command

        so be careful!

 config ELF2FLT_VERSION

     string

     default "head" if ELF2FLT_CVSHEAD

 # Don't remove next line

 # CT_INSERT_VERSION_STRING_BELOW

 config ELF2FLT_EXTRA_CONFIG_ARRAY

     string

     prompt "elf2flt extra config"

     default ""

     help

       Extra flags passed onto ./configure when configuring

       You can enter multiple arguments here, and arguments can contain spaces

       if they are properly quoted (or escaped, but prefer quotes). Eg.:

           --with-foo="1st arg with 4 spaces" --with-bar=2nd-arg-without-space

 endif