arch-hsail: changed gen.py shebang from python(3) to python2.7

arch-hsail: changed gen.py shebang from python(3) to python2.7

gen.py includes code_formatter from m5.util. code_formatter uses the
python2 __metaclass__ attribute, which is ignored by python3, causing
the code_formatter.pattern attribute to be unset.

This prevented scons from building against HSAIL_X86

Signed-off-by: Ryan Gambord
<gambordr@oregonstate.edu>

Change-Id: I5a8bf9e730fd629eb7f9a7ac2dce928235a0dae4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17008
Reviewed-by: Andrea Mondelli <Andrea.Mondelli@ucf.edu>
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>