[OpenMP][libomptarget] rework of fatal error reporting

Summary:
Removed the function that used a lock and varargs
Used the same mechanism as for debug messages

Reviewers: ABataev, gtbercea, grokos, Hahnfeld

Reviewed By: gtbercea, Hahnfeld

Subscribers: mikerice, ABataev, RaviNarayanaswamy, guansong, openmp-commits

Differential Revision: https://reviews.llvm.org/D51226

llvm-svn: 340767