fat_file: Fix `new_from_machos`
fat_file: Fix `new_from_machos`

This now calculates alignments correctly, and pads the resulting
Fat Mach-O so that individual Mach-O slices are page-aligned.

Homebrew/ruby-macho,woodruffw/ruby-macho,Homebrew/ruby-macho,woodruffw/ruby-macho,Homebrew/ruby-macho,woodruffw/ruby-macho