ObjectFileMachO: Silence signed/unsigned comparison warning

File::SeekFromStart returns an off_t representing the position of the
file after seeking.  This return value is always going to be one of two
values: the input or -1 in the case of failure.

ObjectFileMachO compares an expression of type off_t from the return of
File::SeekFromStart(segment.fileoff) and compares it for equality with
segment.fileoff.

The type of segment_command_64::fileoff is unsigned while off_t is
signed, comparing them emits a diagnostic under GCC.

Instead, we can just compare SeekFromSTart with -1 to see if we
successfully seeked.

Differential Revision: http://reviews.llvm.org/D4634