Allow using TLSDESC TLS access for ELF on x86 and riscv

#### 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. https://github.com/riscv-non-isa/riscv-elf-psabi-doc/issues/94 links a bunch of useful documents. Also cc https://github.com/rust-lang/rust/pull/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.