[lld] [lld][WebAssembly] Reset context object after each link (PR #78770)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay approved this pull request.

LGTM as this seems urgent.

wasm should have a unittest similar to `unittests/AsLibELF/SomeDrivers.cpp` for library users.

https://github.com/llvm/llvm-project/pull/78770