kernel: add mingw

Add the option to build a cross-compiler for kernel type 'mingw'.
The resulting cross-compiler can be used to build applications on a Linux host
that can be run on a Windows target.

Compiler is build using the mingwrt and w32-api packages aviable from the
MinGW project (http://sourceforge.net/projects/mingw).

The windows headers (w32-api package) are extracting with the kernel_headers
step The libraries and other headers from both packages are build and
installed in the various steps of libc

Signed-off-by: Bart vdr Meulen <bartvdrmeulen@gmail.com>
[yann.morin.1998@anciens.enib.fr: fix kernel headers comment, don't "return 0"]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # Compute powerpc-specific values

 CT_DoArchTupleValues () {

     # The architecture part of the tuple, override only for 64-bit

     if [ "${CT_ARCH_64}" = "y" ]; then

         CT_TARGET_ARCH="powerpc64"

     fi

     # Add spe in the tuple if needed

     case "${CT_LIBC},${CT_ARCH_POWERPC_SPE}" in

         glibc,|eglibc,)   CT_TARGET_SYS=gnu;;

         glibc,y|eglibc,y) CT_TARGET_SYS=gnuspe;;

     esac

     # Add extra flags for SPE if needed

     if [ "${CT_ARCH_POWERPC_SPE}" = "y" ]; then

         CT_ARCH_TARGET_CFLAGS="-mabi=spe -mspe"

         CT_ARCH_CC_CORE_EXTRA_CONFIG="--enable-e500_double"

         CT_ARCH_CC_EXTRA_CONFIG="--enable-e500_double"

     fi

 }