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

[Kelvin Li via cfe-commits]

kkwli wrote:

This commit breaks `flang/test/Integration/complex-div-to-llvm.f90` on AIX. For example,

```fortran
subroutine f(x)
  complex(8) :: x, y
  y = x
end
```
emits:
```
define void @f_(ptr noalias %0) #0 {
  %2 = alloca { double, double }, i64 1, align 8
  %3 = load { double, double }, ptr %0, align 4
  store { double, double } %3, ptr %2, align 4
  ret void
}
```
The `align` on the `load` and `store` becomes `4` instead `8`. #174016 

@nikic I want to confirm if it is the expected output.

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