wasm-unknown target shouldn't set `--import-memory` by default

[jmillikin]

The wasm-unknown target invokes wasm-ld with the --import-memory option, which causes the following import to be emitted in the output:

(import "env" "memory" (memory $mimport$0 2))

This is undesirable to have as default behavior for wasm-unknown, because it introduces a mandatory import into what is otherwise a freestanding module.

If the --import-memory were to be removed from targets/wasm-unknown.json ldflags, then wasm-ld would emit this instead:

;; Declare and export the Memory, which requires no special import binding in the host
(memory $0 2)
(export "memory" (memory $0))

This option appears to have been added as part of a larger change in 2cd966d, but it's unclear from the commit message why --import-memory was added. Based on discussion in the PR, I think it was simply to reduce the number of bytes in an absolute bare-bones "hello world" module, which IMO is not a compelling reason to have it set by default.

[aykevl]

Because wasm-unknown by default doesn't target a particular platform, it's difficult to say whether memory should be imported or exported by default.
@deadprogram what do environments like wazero expect? Memory, or no memory? Or do they support both?

[deadprogram]

I think @jmillikin is probably correct in what the default should be.

[deadprogram]

See #4414 for a correction.