scons: Use the generalized switching headers on the GPU ISA.

scons: Use the generalized switching headers on the GPU ISA.

Now that the switching header implementation has been generalized, there's
no need to have two nearly identical implementations for the two different
groups of headers.

Change-Id: Ie7c24fcddbc672ac5ca2d69bfc35696f42c55580
Reviewed-on: https://gem5-review.googlesource.com/2984
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-by: Tony Gutierrez <anthony.gutierrez@amd.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>