[clang] [PowerPC] Fix incorrect store alignment for __builtin_vsx_build_pair() (PR #108606)

[Nemanja Ivanovic via cfe-commits]

nemanjai wrote:

Will this affect the code that is generated for ‘__vector_quad’ values? There is no direct way to load and store those so they will use pair loads and stores. 

https://github.com/llvm/llvm-project/pull/108606