RISCV: Add COFF address space

PE spec defines and reserves to following for RISCV

IMAGE_FILE_MACHINE_RISCV32  0x5032
IMAGE_FILE_MACHINE_RISCV64  0x5064
IMAGE_FILE_MACHINE_RISCV128 0x5128

https://msdn.microsoft.com/en-us/library/windows/desktop/ms680547(v=vs.85).aspx

Reviewers: asb, rnk, compnerd

Differential Revision: https://reviews.llvm.org/D41571

llvm-svn: 325667