[ARM] Adding .arch directives around WMMX unwind code

[ARM] Adding .arch directives around WMMX unwind code

Some unwind code is purposedly old enough to work on previous architecutres
and they're guaranteed to never trigger in newer architectures, so we need
to add .arch directives to tell the compiler/assembler that it's fine and we
know what we're doing.

Fixes PR29149.

llvm-svn: 279871