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>

 # Kernel options

 menu "Operating System"

 # Config option used throughout the config and code to determine wether

 # we have a kernel or not (there might be different bare metal stuff)...

 config BARE_METAL

     bool

 config MINGW32

     bool

 # Each target OS (aka kernel) that support shared libraries can select

 # this, so the user can decide whether or not to build a shared library

 # enabled toolchain

 config KERNEL_SUPPORTS_SHARED_LIBS

     bool

 config KERNEL

     string

 config KERNEL_VERSION

     string

 source "config.gen/kernel.in"

 comment "Common kernel options"

 config SHARED_LIBS

     bool

     prompt "Build shared libraries"

     depends on KERNEL_SUPPORTS_SHARED_LIBS

     default y

     help

       Say 'y' here, unless you don't want shared libraries.

       You might not want shared libraries if you're building for a target that

       don't support it (maybe some nommu targets, for example, or bare metal).

 source "config.gen/kernel.in.2"

 endmenu