[lipo] Support creating Universal 64 bit Mach-O files. (#67737)

Xcode `lipo` seems to support a non-documented `-fat64` option that
creates Universal Mach-O archives using 64 bit versions of the
`fat_arch` header, which allows offsets larger than 32 bits to be
specified.

Modify `llvm-lipo` to support the same flag, and use the value of the
flag to use either 32 bits or 64 bits Mach-O headers.

The Mach-O universal writer allows specifying a new option to write
these 64 bits headers. The default is still using 32 bits.

`dsymutil` implemented support for a similar flag in
https://reviews.llvm.org/D146879.