kinst: simplify trampoline fill definitions

Centralize KINST_TRAMP_FILL_PATTERN and KINST_TRAMP_FILL_SIZE to reduce
redefinitions, and use the architecture-dependent kinst_patchval_t as
their size.

Reviewed by: markj
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40406