arch/sparc: add absic support

Add support for building SPARC targeted toolchain.
With this patch I have built a working sparc V8 (32 toolchain).

Testing shows that not all gcc versions works well:
4.4.1 OK (kernel builds and the final kernel can boot)
4.4.2 Not tested
4.4.3 Not tested
4.4.4 BAD (Kernel can build but fails during boot)
4.4.5 BAD (Kernel can build but fails during boot)
4.5.1 BAD (Build fails with a spill related ICE - http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35664)
4.5.2 OK (kernel builds and boots)

I have successfully been using the 4.5.2 version for a few months.

This patch does not add support for the LEON variant.
That may come later.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
[yann.morin.1998@anciens.enib.fr: for 32-bit, default CT_TARGET_ARCH is OK]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # s390 specific config options

 # depends on EXPERIMENTAL

 config ARCH_s390

     select ARCH_SUPPORTS_32

     select ARCH_SUPPORTS_64

     select ARCH_DEFAULT_32

     select ARCH_USE_MMU

     select ARCH_SUPPORT_FPU

     help

       Defined by IBM in:

         32-bit ESA/390       : http://publibfp.boulder.ibm.com/cgi-bin/bookmgr/BOOKS/dz9ar008/CCONTENTS

         64-bit z/Architecure : http://publibfp.boulder.ibm.com/cgi-bin/bookmgr/BOOKS/dz9zr002/CCONTENTS