meson.build: wire loongson64_tsd
Enable loongarch64_tsd for Meson, which is originally only handled in configure.ac .
Depends on !285 (closed) and the second patch plays the same role with it for Meson.
Previously a commit is here to workaround broken linker relaxation of Binutils 2.41 on LoongArch, but it's temporarily dropped because it's proven that linker relaxation is not the fault on RISC-V.
Edited by Icenowy Zheng