Scan inlinable methods for references to global variables and
    functions (forgot to do that earlier).
    
    Track all packages mentioned by exports (that should have been done
    earlier too).
    
    Record assembler name in export data, so that we can inline calls to
    non-Go functions.  Modify gccgoimporter code to skip assembler name.
    
    This increases the number of inlinable functions in the standard
    library from 215 to 439.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/180677

From-SVN: r271976