[dsymutil] Rewrite thumb triple names in user visible messages.

We autodetect triples from the input file(s) while reading the mach-o debug map.
As we need to create a Target from those triples, we always chose the thumb
variant (because the arm variant might not be 'instantiable' eg armv7m). The
user visible architecture names should still be 'arm' and not 'thumb' variants
though.

llvm-svn: 245960