Avoid double bundling underscore

Summary:
Prevent underscore in kmath from being bundled in when we
have a global underscore.

See 2233e1b32f8e8eb71e421fe1d3c442325cd9c1ed for more discussion.

Test Plan:

    cd kmath && make install && cd ..
    make build
    wc -l build/perseus-2.js

See about 38k lines instead of about 40k lines.

Auditors: joel