[llvm-strip][WebAssembly] Support strip flags

Summary:
Add support for the basic section stripping (and keeping) flags for wasm:
strip with no flags, --strip-all, --strip-debug,
--only-section, --keep-section, and --only-keep-debug.

Factor section removal into a function and use a predicate chain like
the ELF implementation.

Reviewers: jhenderson, sbc100

Differential Revision: https://reviews.llvm.org/D73820