The psABI work is not finalized, but based on the current status of
https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/373, I think
the attached is a valid (but untested) implementation of TLSDESC for
riscv64. Actually activating it requires also adding the relocation
type macro to riscv64/reloc.h.

If any rv folks could look it over and make sure I haven't made any
stupid asm errors or missed any obvious optimizations, that would help
to quickly get this merged when the psABI is finalized.

Rich