i#3348 sym conflicts: fix Mac drlibc link error (#3412)
i#3348 sym conflicts: fix Mac drlibc link error (#3412)

Adds a weak d_r_strncmp in drlibc to fix a Mac link error in non-core
uses of drlibc.

Issue: #3348
AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio