Update CORE-V documentation.

Created by: jeremybennett

* doc/extend.texi (CORE-V Builtin-in Functions): Rename as RISC-V (CORE-V), provide sub-subsection for the builtins for each ISA extension, present builtins as simple lists, reorder ISA extensions alphabetically.
* doc/invoke.texi (RISC-V Options): Add missing CORE-V options for -march and order those options alphabetically.