Re-enable Fuchsia CI target

[seanmonstar]

The Fuchsia CI target was disabled, due to this message (seen here):

toolchain 'stable-x86_64-unknown-linux-gnu' does not contain component 'rust-std' for target 'x86_64-unknown-fuchsia'

I don't know why, or what is needed to fix it, but it should likely be fixed.

[pranavek]

@seanmonstar I checked the Rust platform support page and figured out that the Fuchsia target name specified in CI is not listed there. x86_64-fuchsia seems to be the closest one. Opened a PR #109 to re-enable Fuchsia build.