[OpenMP][Bugfix] Move double and float versions of abs under c++ macro

Summary:
This is a fix for the reported bug:

[[ https://bugs.llvm.org/show_bug.cgi?id=41861 | 41861 ]]

abs functions need to be moved under the c++ macro to avoid conflicts with included headers.

Reviewers: tra, jdoerfert, hfinkel, ABataev, caomhin

Reviewed By: jdoerfert

Subscribers: guansong, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61959

llvm-svn: 360809