try_compile: Don't accept try_run arguments

Modify cmCoreTryCompile to only recognize try_run arguments when actually parsing try_run. (The old behavior was to recognize try_run arguments for try_compile also and then complain.)

This has a small chance that a try_run keyword will be consumed as part of a multi-valued keyword argument. However, this behavior is more consistent with other commands, as we don't normally treat keywords as universally reserved. Also, the code is noticeably simplified.