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>

# ARM specific configuration file

config ARCH_ARM_MODE

    string

    default "arm"   if ARCH_ARM_MODE_ARM

    default "thumb" if ARCH_ARM_MODE_THUMB

choice

    bool

    prompt "Default instruction set mode"

    default ARCH_ARM_MODE_ARM

config ARCH_ARM_MODE_ARM

    bool

    prompt "arm"

    help

      Defaults to emitting instructions in the ARM mode.

config ARCH_ARM_MODE_THUMB

    bool

    prompt "thumb (EXPERIMENTAL)"

    depends on EXPERIMENTAL

    help

      Defaults to emitting instructions in the THUMB mode.

endchoice

config ARCH_ARM_INTERWORKING

    bool

    prompt "Use Thumb-interworking (READ HELP)"

    depends on EXPERIMENTAL

    help

      Excerpt from the gcc manual:

      > Generate code which supports calling between the ARM and Thumb

      > instruction sets. Without this option the two instruction sets

      > cannot be reliably used inside one program. The default is

      > [not to use interwork], since slightly larger code is generated

      > when [interwork] is specified.

config ARCH_ARM_EABI

    bool

    prompt "Use EABI"

    default y

    help

      Set up the toolchain so that it generates EABI-compliant binaries.

config ARCH_ARM_ABI_OK

    bool

    default y

    depends on ! ARCH_ARM_EABI

    select ARCH_SUPPORT_ABI