compiler, runtime: copy string code from Go 1.7

    
    Add compiler support for turning concatenating strings into a call to
    a runtime function that takes the appropriate number of arguments.
    
    Rename some local variables in mgc0.c to avoid macros that the new
    rune.go causes to appear in runtime.inc.
    
    Reviewed-on: https://go-review.googlesource.com/30827

From-SVN: r241074