[llvm] [PowerPC][AIX] Support ByVals with greater alignment then pointer size (PR #93341)

[Sean Fertile via llvm-commits]

mandlebug wrote:

> Implementation is NOT compatible with XL. Instead it handles all ByVals with greater alignment then pointer width the same way XL handles Byvals that have vector members. For overaligned objects that do not contain vectors XL does not align them properly if they are passed in the GPR argument registers.
> 
> This patch was originally written by Sean Fertile @mandlebug.
> 
> Previously on Phabricator https://reviews.llvm.org/D105659

Worth noting this approach is compatible with gcc on AIX (as well as OpenXL)

https://github.com/llvm/llvm-project/pull/93341