[RISCV] Define RISC-V specific e_flags

Add RISC-V e_flags as defined in the ABI document:

https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md#file-header

Differential Revision: https://reviews.llvm.org/D38310
Patch by Chih-Mao Chen.

llvm-svn: 314386