[llvm-debuginfo-analyzer][DOC] Change .wasm references to .o (#85566)

As part of the WebAssembly support work
https://github.com/llvm/llvm-project/pull/82588

As the object files used in the test cases are a single object (just
produced by clang without being processed by wasm-ld), it was determined
to use .o intead of .wasm.

Update the README.txt to reflect that the tool now supports WebAssembly.