ARM: support interrupt attribute

This function-attribute modifies the callee-saved register list and function
epilogue (specifically the return instruction) so that a routine is suitable
for use as an interrupt-handler of the specified type without disrupting
user-mode applications.

rdar://problem/14207019
[SystemZ] Extend pseudo conditional 8- and 16-bit stores to high words

As the comment says, we always want to use STOC for 32-bit stores.