[dsymutil] Fix -arch option for thumb variants.

r267249 removed the dual ARM/Thumb interface from MachOObjectFile,
simplifying llvm-dsymutil's code. This unfortunately also regressed
llvm-dsymutil's ability to select thumb slices, because the simplified
code was also dealing with the discrepency between the slice arch
(eg. armv7m) and the triple arch name (eg. thumbv7m).

llvm-svn: 268894