RISC-V -menable-experimental-extensions option

[Jim Wilson]

I'm not aware of any other target that has a similar feature, so I thought
a bit of discussion first might be useful.

For most ISAs, there is one organization that owns it, and does development
internally, in private.  For RISC-V, the ISA is owned by RISC-V
International which has no developers.  The development all happens
externally, in public, spread across at least a dozen different
organizations.  So we have the problem of coordinating this work,
especially for draft versions of extensions.  So we would like to add
support for draft extensions to mainline, controlled by a
-menable-experimental-extensions option.  For features enabled by this
option, there would be no guarantee that the next compiler release is
compatible with the previous one, since the draft extension may change in
incompatible ways.

LLVM already has support for this option.
    http://lists.llvm.org/pipermail/llvm-dev/2020-January/138364.html
    https://reviews.llvm.org/D73891

We are still discussing the details of how this will work.  We may want to
limit this to 'stable" draft extensions, and put the unstable drafts on a
vendor branch.

We have been doing work on branches in the github.com riscv tree, but there
are issues with tracking who has copyright assignments, issues with
identifying who exactly a github user actually is, and issues with getting
the right set of people right access to the trees.  These won't be problems
if we are using the FSF trees instead.

We want this draft extension support on mainline for the same reasons that
the LLVM developers do, to ensure that everyone is working in the same
branch in the upstream tree.  And it is easiest to do that if that branch
is mainline.

This is just a binutils and gcc proposal at the moment, but we might need
something on the gdb side later, like a
  set riscv experimental-extensions 1
or whatever command to enable support for draft extensions.

Jim