Accessing result data of target options without Mask or Var properties

[The Other]

Hi,
How would I access the result data of target options that don't have Mask
or Var properties? For example, how would I access the result ISA string in
the -march option for the RISC-V target?

Here is the relevant option code inside the .opt file:
march=
Target Report RejectNegative Joined
-march= Generate code for given RISC-V ISA (e.g. RV64IM).  ISA strings must
be
lower-case.

I assume that this information is stored somewhere, as I read somewhere
online that GCC generates different assembly for different ISA strings,
though I suppose this could be referring to previous versions of GCC. I am
attempting to access this information for use in a target hook.

Thanks,
Theo