Fix wasm32-unknown-unknown by passing -c

[NobodyXu]

Fix #1423

[NobodyXu]

I don't know why it's removed, cc @madsmtm do you remember why?

[madsmtm]

Traced this back to #1322.

[madsmtm]

Perhaps we need to add an (internal) flag needs_linker to is_flag_supported? And set it to true for -fembed-bitcode=... (and maybe others)?

I generally agree that we should not attempt to link if we can avoid it, so we really should pass -c unless absolutely necessary that we don't.

[madsmtm]

Hmm, actually, I think #1322 is wrong, and the real solution was #1379.

CC @clubby789, can you check if this PR still works for you?

[clubby789]

Hmm, actually, I think 1322 is wrong, and the real solution was 1379.

CC @clubby789, can you check if this PR still works for you?

1379 does indeed fix the specific case, but 1322 is about preventing the general case (of linker-specific flags being incorrectly reported as supported). The needs_linker idea sounds like a sensible fix to me

[madsmtm]

But cc generally doesn't link, and thus we shouldn't be passing flags that affect the linker in the first place?