[RISCV] Add support for interrupt attribute

 [RISCV] Add support for interrupt attribute

Summary:
Clang supports the GNU style ``__attribute__((interrupt))`` attribute  on RISCV targets.
Permissible values for this parameter are user, supervisor, and machine.
If there is no parameter, then it defaults to machine.
Reference: https://gcc.gnu.org/onlinedocs/gcc/RISC-V-Function-Attributes.html
Based on initial patch by Zhaoshi Zheng.

Reviewers: asb, aaron.ballman

Reviewed By: asb, aaron.ballman

Subscribers: rkruppe, the_o, aaron.ballman, MartinMosbeck, brucehoult, rbar, johnrusso, simoncook, sabuasal, niosHD, kito-cheng, shiva0217, zzheng, edward-jones, mgrang, rogfer01, cfe-commits

Differential Revision: https://reviews.llvm.org/D48412

llvm-svn: 338045