Enable `limit_rdylib_exports` on wasm targets

This commit updates the target specification of wasm targets to set the
`limit_rdylib_exports` value to `true` like it is on other native
platforms. This was originally not implemented long ago as `wasm-ld`
didn't have options for symbol exports, but since then it's grown a
`--export` flag and such to control this. A custom case is needed in the
linker implementation to handle wasm targets as `wasm-ld` doesn't
support linker scripts used on other targets, but other than that the
implementation is straightforward.

The goal of this commit is enable building dynamic libraries on
`wasm32-wasip2` which don't export every single symbol in the Rust
standard library. Currently, without otherwise control over symbol
visibility, all symbols end up being exported which generates
excessively large binaries because `--gc-sections` ends up doing nothing
as it's all exported anyway.

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