ARM: fix mismatch between Clang and backend on exception type.

"-arch armv7k" is only normally used with a watchos target, but Clang doesn't
stop you from giving a "-miphoneos-version-min" option too, converting the
triple to "thumbv7k-apple-ios". In this case the backend will decide to use
SjLj exceptions, so Clang needs to agree so it can create the correct
predefines.

Fortunately, there's a handy function to make the decision for us now.

llvm-svn: 253355