[roll] Roll fuchsia [kernel] Template KTrace on BufferMode

This CL effectively adds a compile-time feature-flag, behind which
we can start landing code to support per-CPU buffers in KTrace.
Concretely, this change templates the KTrace class on a new
BufferMode enum, which currently defaults to BufferMode::kSingle,
which corresponds to the single-buffer mode that exists today.

The naive approach of just templating the existing KTrace singleton
on the new enum would unfortunately require propagating the template
parameter (or just <> in the case of a default) to all callsites,
including all macros. To avoid what is effectively a large find and
replace across a bunch of files, this CL instead renames the KTrace
class to KTraceImpl and templates that class instead. KTrace is
then declared as an alias to KTraceImpl<BufferMode::kSingle>, thus
preserving existing behavior.

Making KTraceImpl templated on BufferMode made defining the static
instance in ktrace.cc a bit challenging, so it's been moved into
ktrace.h as a static inline variable. This should have no functional
effect, and removes the need for an out-of-line templated
initialization (which I couldn't get to work).

All existing KTrace methods were then modified to be template
specializations for the BufferMode::kSingle template argument. As
part of this, all method definitions that were previously in the
header (Init, ReadUser, Reserve) have been moved to the cpp file.
I could put the specialization in the header, but it seemed a bit
cleaner to do in the cpp file.

Note: This CL does not implement any of the per-CPU code; those
template specializations will come in a follow-up CL.

Finally, this CL performs a small refactor of the Control and Start
method, and adds helpers for the Stop and Rewind methods. This
refactor will make it simpler to implement their specializations
for the per-CPU case.

All of this templating will be removed and deleted once per-CPU
buffering is enabled by default.

Original-Bug: 404539312
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1244924
Original-Revision: 073730dc0bd1e463dc83f2e7967a290d41c26263
GitOrigin-RevId: 618ff2796709a7a35b409f7205bc30af1639abd8
Change-Id: I8d87837ef32279f666cce5888de8ec38c0aa051f