[OPENMP][NVPTX]Emit shared memory buffer for reduction as 128 bytes

buffer.

Seems to me, nvlink has a bug with the proper support of the weakly
linked symbols. It does not allow to define several shared memory buffer
with the different sizes even with the weak linkage. Instead we always
use 128 bytes buffer to prevent nvlink from the error message emission.

llvm-svn: 349540