Bring in https://golang.org/cl/135297 from the gc repository to fix a
    GCC bug report.
    
    Original CL description:
    
        The GNU assembler for ARM treats @ as a comment character, so section
        types must be written using % instead.
    
        Fixes https://gcc.gnu.org/PR87260.
    
    Reviewed-on: https://go-review.googlesource.com/135360

From-SVN: r264330