When parsing isa specific values we were accidentally discarding the
value of the flag, and treating it always as a boolean flag.
This would cause a `clif-util` invocation such as
`cargo run -- compile -D --set has_sse41=false --target x86_64 test.clif`
to be interpreted as `--set has_sse41` and enable that feature instead
of disabling it.
fc72b7ccd3