Makefile.asm: Simplify variable expansions in generated headers

The :R:S expressions removed the .pl extension only to add it back
again, so just trim them to using :T alone.

Reviewed by: Pierre Pronchery <pierre@freebsdfoundation.org>, markj, emaste
Differential Revision: https://reviews.freebsd.org/D41588

(cherry picked from commit 7406b6f9761c35eb53f91f54d1a687d96d638907)