Add some javascript special cases to ghc-toolchain

On javascript there isn't a choice of toolchain but some of the
configure checks were not accurately providing the correct answer.

1. The linker was reported as gnu LD because the --version output
   mentioned gnu LD.
2. The --target flag makes no sense on javascript but it was just
   ignored by the linker, so we add a special case to stop ghc-toolchain
   thinking that emcc supports --target when used as a linker.