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>

 # x86 specific options

 ## select ARCH_SUPPORTS_32

 ## select ARCH_SUPPORTS_64

 ## select ARCH_DEFAULT_32

 ## select ARCH_USE_MMU

 ## select ARCH_SUPPORT_ARCH

 ## select ARCH_SUPPORT_CPU

 ## select ARCH_SUPPORT_TUNE

 ##

 ## help The x86 architecture, as defined by:

 ## help     32-bit (ia32) : http://www.intel.com/

 ## help     64-bit (amd64): http://www.amd.com/us-en/Processors/DevelopWithAMD/0,,30_2252_875_7044,00.html