Prioritize using a segment with the name TEXT instead off fileoff 0

lldb needs to find the virtual address of the mach header of a
binary.  It first scans for a segment which starts at file offset
0, and uses the vmaddr of that segment.  If no segment starts at
fileoff 0, it looks for a segment named __TEXT.

This patch changes the order of those, to first search for the TEXT
segment.  We have a situation where binaries exist that have the
DATA segment first, which does not have the vmaddr of the mach header,
it merely happens to come first in the binary file.  It's an unusual
arrangement, but not breaking any rules of Mach-O.  So lldb needs
to handle this.

Differential Revision: https://reviews.llvm.org/D150239
rdar://109128418