Allow using TLSDESC TLS access for ELF on x86 and riscv

[bjorn3]

Feature

See title

Benefit

TLSDESC is a more efficient alternative to the traditional __tls_get_addr that is used for the global-dynamic tls model. TLSDESC is already mandatory on arm64. They were introduced relatively recently on x86 (2008, but with a glibc bugfix in 2024) and riscv (2024), so we should keep them optional for the time being. riscv-non-isa/riscv-elf-psabi-doc#94 links a bunch of useful documents. Also cc rust-lang/rust#132480

Implementation

Add the required relocation types and adapt the TLS access binemit code as relevant.

Alternatives

Not support this and accept slower TLS accesses.