ARM: diagnose ARM/Thumb assembly switches on CPUs only supporting one.

Some ARM CPUs only support ARM mode (ancient v4 ones, for example) and some
only support Thumb mode (M-class ones currently). This makes sure such CPUs
default to the correct mode and makes the AsmParser diagnose an attempt to
switch modes incorrectly.

rdar://14024354

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183710 91177308-0d34-0410-b5e6-96231b3b80d8