compiler: rewrite compiler directive support

compiler: rewrite compiler directive support

    Rewrite the compiler directive support to recognize all the compiler
    directives implemented by the current gc compiler.  The directives other
    than go:linkname are now turned into GOPRAGMA flags attached to a
    function or function declaration.  The go:linkname directive is turned
    into a map attached to the Lex object.  No new directives are actually
    implemented yet, they are just recognized.

    Reviewed-on: https://go-review.googlesource.com/26610

From-SVN: r239282