This is to support the memory routines vec_malloc, vec_calloc, vec_realloc, and vec_free. These routines manage memory that is 16-byte aligned. And they are only available on AIX.
The change in TargetLibraryInfoTest.cpp is kind of for testing. I checked previous similar patches, ie, adding platform specific functions, there was no additional test cases added other than adding to TargetLibraryInfoTest.cpp.