Toolchain conventions: the -march string

Hi all,

As Palmer recently highlighted, we have a difference in the current handling
of the -march string in the tools and the ISA naming string as it is defined in
the RISC-V ISA manual.

Given the profiles support in the tools will add further functionality
to the -march string,
it is reasonable to rework the -march string specification in the
RISC-V toolchain
conventions repo. I made a list of change requests to the -march string handling
and un(der)documented behavior and created a first draft, which can be
found here:
Define -march string by cmuellner · Pull Request #26 · riscv-non-isa/riscv-toolchain-conventions · GitHub

If you are interested, please participate in the discussion in the PR.

Note that I don’t want to make this more complex than necessary.
The primary intent is to avoid big documentation holes for one of the
main compiler flags.