[flang] Add runtime interface for SYSTEM_CLOCK

SYSTEM_CLOCK may take up to 3 optional parameters, all of which are
INTENT(OUT). The COUNT and COUNT_MAX parameters are integer scalars,
while COUNT_RATE may be a real or integer scalar.

This patch breaks up the interface into 3 different functions, one for
each parameter. All 3 return integers. It is up to lowering to convert
the results to the preferred type.

Differential Revision: https://reviews.llvm.org/D104851