cmd/internal/obj/ppc64: make assembler almost concurrency-safe

CL 39922 made the arm assembler concurrency-safe.
This CL does the same, but for ppc64.
The approach is similar: introduce ctxt9 to hold
function-local state and thread it through
the assembler as necessary.

One race remains after this CL, similar to CL 40252.

That race is conceptually unrelated to this refactoring,
and will be addressed in a separate CL.

Passes toolstash-check -all.

Updates #15756

Change-Id: Icc37d9a971bed2184c8e66b1a64f4f2e556dc207
Reviewed-on: https://go-review.googlesource.com/40372
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>