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>

# powerpc specific configuration file

config ARCH_POWERPC_SPE

    bool

    prompt "Enable SPE support"

    help

      Add support for the Signal Processing Engine.  This will set up

      the toolchain so that it supports the SPE ABI extensions. This

      mainly targets Freescale e500 processors.

      Setting this option will append "spe" to the end of your target

      tuple name (e.g., powerpc-e500v2-linux-gnuspe) so that the gcc

      configure/build system will know to include SPE ABI support. It

      will also automatically add "-mabi=spe -mspe" to your TARGET_CFLAGS,

      and "--enable-e500_double" to your CC_EXTRA_CONFIG_ARRAY, so you

      do not need to explicitly add them.