[lldb] Support Universal Mach-O binaries with a fat64 header

Support universal Mach-O binaries with a fat64 header. After
4d683f7f, dsymutil can now generate such binaries when the offsets
would otherwise overflow the 32-bit offsets in the regular fat header.

rdar://107289570

Differential revision: https://reviews.llvm.org/D147012