Merge lld/ELF's RISC-V TLSDESC into release/18.x

[MaskRay]

Ensure that linker support (#79239) is in place before more vendors enable TLSDESC for RISC-V.

/cherry-pick ccb99f2 849951f 1117fdd

@ilovepi

[github-actions]

/pull-request #79509

[llvmbot]

@llvm/issue-subscribers-lld-elf

Author: Fangrui Song (MaskRay)

Ensure that linker support (https://github.com//pull/79239) is in place before more vendors enable TLSDESC for RISC-V.

/cherry-pick ccb99f2 849951f 1117fdd

@ilovepi

[tstellar]

Merged: e2521ea 3d02473 e9d99e5