util: Delete the build_cross_gcc utility.

Now that crosstool-ng can be used for any of the supported ISAs, the
build_cross_gcc tool is no longer needed.

Change-Id: Ie72d84197662eb0bea03ec27830b3ad4dbaa3084
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/41454
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabe.black@gmail.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
7 files changed