Comment reproduced here:

Note the following u64 alignments:
  x86-linux:   4
  x86-windows: 8
LLVM makes x86_fp80 have the following alignment and sizes regardless
of operating system:
  x86_64: size=16, align=16
  x86:    size=12, align=4
However in Zig we override x86-windows to have size=16, align=16
in order for the property to hold that u80 and f80 have the same ABI size.

Fixes "error: destination type 'f80' has size 12 but source type 'u80'
has size 16" when trying to bitcast between f80 and u80 on i386-windows.