[cmake] Add support to build native executables

Summary: As per title. This ensures that we can build the executable used to generate various assets.

Test Plan:

cmake -GNinja .. -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Win32.cmake -DBUILD_BITCOIN_QT=OFF -DBUILD_BITCOIN_SEEDER=OFF
ninja generate_univalue_escapes_h

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1252