libc/newlib: allow using CVS snapshots

The newlib "team" rolls new releases about once a year (december).
This is quite a long time between releases, in case code was fixed.
So, allow user to use a CVS snapshot to benefit early from fixes
and enhancements to newlib.

 # x86 specific options

 config ARCH_x86

     select ARCH_USE_MMU

     select ARCH_SUPPORT_ARCH

     select ARCH_SUPPORT_CPU

     select ARCH_SUPPORT_TUNE

     help

       The x86 architecture, as defined by:

         http://www.intel.com/