fix: add libc fields only to platforms that have libc

[yisibl]

Add the libc field in a clearer and more explicit way.
After this PR, linux-gnueabihf also no longer adds the libc field.

Before

{
  "name": "@parcel/css-linux-arm-gnueabihf",
  "libc": [
    "gnueabihf"
  ]
}

Related PRs: #205, #208

[Brooooooklyn]

@yisibl the libc field on linux-arm-gnueabihf should be glibc.

[yisibl]

@Brooooooklyn Thanks! It looks like the napi-rs template needs to be updated as well?

[Brooooooklyn]

@yisibl I don't think the libc field in linux-arm-gnueabihf is necessary, since there is only glibc runtime for linux-arm.

[yisibl]

@devongovett PTAL, thanks!