util: Add a script to help build cross compilers.

util: Add a script to help build cross compilers.

Cross compilers are very useful when working with gem5. The how-to this
script is based on assumed the compiler was targeting linux, so there
isn't any support for compilers targeting other or no OS. That might be
possible to add in the future.

Change-Id: I2cb30ecbdd4c6292146ea64940348c24385046f9
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26763
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>