Ability to pass flags to external linker (such as wasm-ld)

[jmillikin]

While looking into #4319, I discovered that there doesn't seem to be a way to pass flags to the wasm-ld external linker from the tinygo build command. The only way I could find is to create a custom target JSON, which is a bit heavyweight.

There is -ldflags, but it's documented as being for flags compatible with the Go linker.

The standard Go build system defines -extldflags which seems to be what I want, but tinygo build doesn't have that flag.

[dgryski]

I can probably take a look at this.