[llvm-readobj] Rename JSON Flag fields to be more consistent

Today the JSON uses `Value` and `RawValue` when printing `Flags`, when really
the `Value` field is always the name of an Enum variant, and `RawValue` is its
underlying numeric value. Similarly, we rename the `RawFlags` key to `Value`,
to match the new scheme. This also allows JSON parsing to use consistent logic
for `Flag` types.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D137091