[OpenMP] Initial implementation of OMP 5.0 Memory Management routines

Implemented omp_alloc, omp_free, omp_{set,get}_default_allocator entries,
and OMP_ALLOCATOR environment variable.

Added support for HBW memory on Linux if libmemkind.so library is accessible
(dynamic library only, no support for static libraries).
Only used stable API (hbwmalloc) of the memkind library
though we may consider using experimental API in future.

The ICV def-allocator-var is implemented per implicit task similar to
place-partition-var.  In the absence of a requested allocator, the uses the
default allocator.

Predefined allocators (the only ones currently available) are made similar
for C and Fortran, - pointers (long integers) with values 1 to 8.

Patch by Andrey Churbanov

Differential Revision: https://reviews.llvm.org/D51232

llvm-svn: 341687