Add llvm-sys linkage flags

[nohenry]

Is your feature request related to a problem? Please describe. Currently, only static linking is supported. It would be nice to be able to dynamically link to LLVM.

Describe the solution you'd like Adding feature flags that align with llvm-sys's force-dynamic, prefer-dynamic, force-static, and prefer-static. Default would be force-static which is currently done anyways.