add TSD dispatch assembly for RISCV64
The code is just a translation of the LoongArch TSD code, ported to verify the linker issue of LoongArch Binutils 2.41. However it should also be useful on RISC-V machines, and verified to work on a SG2042.
Signed-off-by: Icenowy Zheng uwu@icenowy.me
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Depends on !286 (merged) which wires in LoongArch code into meson.build.
Edited by Icenowy Zheng