Alt mechanism to find the first loadable seg in a Mach-O binary

ObjectFileMachO, for a couple of special binaries at the initial
launch, needs to find segment load addresses before the Target's
SectionLoadList has been initialized. The calculation to find
the first segment, which is at the same address as the mach header,
was not correct if the binary was in the Darwin shared cache.
Update the logic to handle that case.

Differential Revision: https://reviews.llvm.org/D119602
rdar://88802629