binutils-2_28-branch: Various RISC-V Fixes

[Palmer Dabbelt]

Here are the three RISC-V patches that aren't on the 2.28 branch but are on
master.  One has been submitted for 2.28, but I haven't gotten any feedback,
while the other two are new.

We'd really like these in the release so we can build Linux with the 2.28
release rather that having to tell the Linux devs to patch it or get binutils
master.

Are these OK for the 2.28 branch?

Also: I'd like to check on the policy for commiting to 2.28.  I haven't gotten
any feedback on my "OK for 2.28?" messages recently and I was told that I
needed an OK for the 2.28 branch from Tristan Gingold.  I haven't seen any mail
from Tristan Gingold recently, but when I went to rebase I saw a bunch of
patches committed.  Am I asking for approval incorrectly?

[PATCH 1/3] Add SFENCE.VMA instruction
[PATCH 2/3] Add new counter-enable CSRs
[PATCH 3/3] bfd: RISC-V: relax to gp in more cases.