remove deprecated register storage class specifier

While importing libomp into the FreeBSD base system we encountered
Clang warnings that "'register' storage class specifier is deprecated
and incompatible with C++1z [-Wdeprecated-register]".

Differential Revision:	https://reviews.llvm.org/D35124


git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@307441 91177308-0d34-0410-b5e6-96231b3b80d8