configure: add support for helper script to compute version string

Some projects are using (or planning to use) crosstool-NG, and are storing
it in their VCS, which might not be Mercurial. At the same time, those
projects may want to track development snapshots versions the way we do
with the Hg identity string (hg id).

Provide a way for these project to do so, without having to patch
./configure, and maintain that patch over-and-over again.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # ARM specific configuration file

 ## select ARCH_SUPPORTS_32

 ## select ARCH_DEFAULT_32

 ## select ARCH_SUPPORTS_BOTH_MMU

 ## select ARCH_DEFAULT_HAS_MMU

 ## select ARCH_SUPPORTS_BOTH_ENDIAN

 ## select ARCH_DEFAULT_LE

 ## select ARCH_SUPPORT_ARCH

 ## select ARCH_SUPPORT_CPU

 ## select ARCH_SUPPORT_TUNE

 ## select ARCH_SUPPORT_FPU

 ## select ARCH_SUPPORT_SOFTFP

 ##

 ## help The ARM architecture, as defined by:

 ## help     http://www.arm.com/