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>

 # Dummy config file to not use a C library *at all*

 ## depends on BARE_METAL

 ##

 ## help Do not use a C library.

 ## help

 ## help This is usefull if your project is self-contained, does not rely on

 ## help an external C library, and provides all the necessary bits.

 ## help

 ## help Most probably usefull to bootloaders, as they generally don't depend

 ## help on the C library.

 ## help

 ## help If unsure: do *not* choose that, and use another option in the choice.