xtools-x86_64: move to gcc 4.7.3

Reviewed By: dustin

For the x86_64 cross-compiling toolchain move to gcc version 4.7.3.

This change is to avoid xtool compilation issues reported by several
users.  Particularly users of Ubuntu 14.04 where the host compiler
is:

  gcc version 4.8.2 (Ubuntu 4.8.2-19ubuntu1)

See the gcc mailing list comments for more information:

  http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57625

Testing Done:

Compile images using Ubuntu 14.04 and installed ONIE on the kvm_x86_64
reference platform and two additional x86 based platforms without
problems.  Ran through the demo OS installer also.

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>
xtools-powerpc: move to gcc 4.7.3

Reviewed By: dustin

For the cross-compiling toolchain move to gcc version 4.7.3.

This change is only to keep pace with the x86 CPU architecture where
the gcc version was bumped to 4.7.3 for more technical reasons.

Testing Done:

Compile images and installed ONIE on the fsl_p2020rdbpca-r0 reference
platform without problems.

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>