Fix a bugs with using some Mach-O command line flags like "-arch armv7m".

The Mach-O command line flag like "-arch armv7m" does not match the
arch name part of its llvm Triple which is "thumbv7m-apple-darwin”.

I think the best way to fix this is to have
llvm::object::MachOObjectFile::getArchTriple() optionally return the
name of the Mach-O arch flag that would be used with -arch that
matches the CPUType and CPUSubType.  Then change
llvm::object::MachOUniversalBinary::ObjectForArch::getArchTypeName()
to use that and change it to getArchFlagName() as the type name is
really part of the Triple and the -arch flag name is a Mach-O thing
for a specific Triple with a specific Mcpu value.

rdar://29663637

llvm-svn: 290001