scripts/crosstool-NG.sh.in: On MacOS/BSD use the output of CT_DoConfigGuess for CT_BUILD.

On 64bit MacOS `gcc -dumpmachine` gives i686 for the host machine.
This conflicts with the expectations of some following configure scripts
that a 64bit x86 is given as x86_64; i686 is understood as a 32 bit machine.
config.guess sets the host machine in CT_BUILD correctly.

yann.morin.1998@anciens.enib.fr:
As suggested by Khem RAJ on the ML, always use config.guess.

# Compute Alpha-specific values

CT_DoArchTupleValues () {

    # The architecture part of the tuple:

    CT_TARGET_ARCH="${CT_ARCH}${CT_ARCH_ALPHA_VARIANT}"

}