[RISCV] Add missing e_flag EF_RISCV_TSO

This flag was defined here https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-elf.adoc

Reviewed By: asb, MaskRay, jhenderson

Differential Revision: https://reviews.llvm.org/D111939