[OpenMP][libomptarget] Refactor SPMD and runtime requirement checking

Summary: Refactor the checking for SPMD mode and whether the runtime is initialized or not. This uses constant flags which enables the runtime to optimize out unused sections of code that depend on these flags.

Reviewers: ABataev, caomhin

Reviewed By: ABataev

Subscribers: guansong, jfb, openmp-commits

Differential Revision: https://reviews.llvm.org/D54960

llvm-svn: 347698