cmd/compile: teach assemblers to accept a Prog allocator

cmd/compile: teach assemblers to accept a Prog allocator

The existing bulk Prog allocator is not concurrency-safe.
To allow for concurrency-safe bulk allocation of Progs,
I want to move Prog allocation and caching upstream,
to the clients of cmd/internal/obj.

This is a preliminary enabling refactoring.
After this CL, instead of calling Ctxt.NewProg
throughout the assemblers, we thread through
a newprog function that returns a new Prog.

That function is set up to be Ctxt.NewProg,
so there are no real changes in this CL;
this CL only establishes the plumbing.

Passes toolstash-check -all.
Negligible compiler performance impact.

Updates #15756

name        old time/op     new time/op     delta
Template        213ms ± 3%      214ms ± 4%    ~     (p=0.574 n=49+47)
Unicode        90.1ms ± 5%     89.9ms ± 4%    ~     (p=0.417 n=50+49)
GoTypes         585ms ± 4%      584ms ± 3%    ~     (p=0.466 n=49+49)
SSA             6.50s ± 3%      6.52s ± 2%    ~     (p=0.251 n=49+49)
Flate           128ms ± 4%      128ms ± 4%    ~     (p=0.673 n=49+50)
GoParser        152ms ± 3%      152ms ± 3%    ~     (p=0.810 n=48+49)
Reflect         372ms ± 4%      372ms ± 5%    ~     (p=0.778 n=49+50)
Tar             113ms ± 5%      111ms ± 4%  -0.98%  (p=0.016 n=50+49)
XML             208ms ± 3%      208ms ± 2%    ~     (p=0.483 n=47+49)
[Geo mean]      285ms           285ms       -0.17%

name        old user-ns/op  new user-ns/op  delta
Template         253M ± 8%       254M ± 9%    ~     (p=0.899 n=50+50)
Unicode          106M ± 9%       106M ±11%    ~     (p=0.642 n=50+50)
GoTypes          736M ± 4%       740M ± 4%    ~     (p=0.121 n=50+49)
SSA             8.82G ± 3%      8.88G ± 2%  +0.65%  (p=0.006 n=49+48)
Flate            147M ± 4%       147M ± 5%    ~     (p=0.844 n=47+48)
GoParser         179M ± 4%       178M ± 6%    ~     (p=0.785 n=50+50)
Reflect          443M ± 6%       441M ± 5%    ~     (p=0.850 n=48+47)
Tar              126M ± 5%       126M ± 5%    ~     (p=0.734 n=50+50)
XML              244M ± 5%       244M ± 5%    ~     (p=0.594 n=49+50)
[Geo mean]       341M            341M       +0.11%

Change-Id: Ice962f61eb3a524c2db00a166cb582c22caa7d68
Reviewed-on: https://go-review.googlesource.com/39633
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>