Revert r344587.

The fasttrap_isa.h header is needed by libdtrace, not just the kernel.

Remove stub fasttrap implementations.

No platforms except i386, amd64 and powerpc implement fasttrap; the
fasttrap files for other arches do not contain any code and bloat
the output from cscope, so just remove them.

MFC after: 1 week

First cut of DTrace for AArch64.

Reviewed by: andrew, emaste
Sponsored by: ARM Limited
Differential Revision: https://reviews.freebsd.org/D2738