[clang] [llvm] [PowerPC][AIX] Specify correct ABI alignment for double (PR #144673)

[Ralf Jung via cfe-commits]

RalfJung wrote:

My understanding is that for this type 8 is the "preferred" alignment but only 4 is "required", so alloca sets 8 to satisfy the preference but load/store cannot set 8 since only 4 is actually required.

https://github.com/llvm/llvm-project/pull/144673