[lld][WebAssembly] Add --keep-section flag

This flag causes wasm-ld preserve a section even in the face of
`--strip-all`.  This is useful, for example, to preserve the
target_features section in the ase of clang (which can run wasm-opt
after linking), and emcc (which performs a bunch of post-link work).

Fixes: https://github.com/llvm/llvm-project/issues/60613
Fixes: https://github.com/llvm/llvm-project/issues/55781

Differential Revision: https://reviews.llvm.org/D149917