tcmode-external-sourcery: Add x86_64-amd-linux-gnu toolchain prefix.

Allow x86-64 builds to use x86_64-amd-linux-gnu as the prefix for the
cross tools.

Signed-off-by: Drew Moseley <drew_moseley@mentor.com>
Merge pull request #31 from drewmoseley/master

tcmode-external-sourcery: Add x86_64-amd-linux-gnu toolchain prefix.