ARM: support interrupt attribute

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191766 91177308-0d34-0410-b5e6-96231b3b80d8