components: Fix support for 0-sized flags

[alexcrichton]

This commit goes through and updates support in the various argument
passing routines to support 0-sized flags. A bit of a degenerate case
but clarified in WebAssembly/component-model#76 as intentional.

[alexcrichton]

cc @dicej as well

[jameysharp]

Looks like a correct implementation of the changed spec to me!

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[alexcrichton]

I pushed up two test fixes for the failed CI:

• One updates make_echo_component to support zero-sized types
• The other updates the Lower structure generated by the various procedural macros to guarantee an alignment that matches ValRaw to satisfy assertions in Wasmtime. (probably not strictly necessary but seems better to be safe here)