runtime, sync: use __atomic intrinsics instead of __sync

runtime, sync: use __atomic intrinsics instead of __sync

    GCC has supported the __atomic intrinsics since 4.7.  They are better
    than the __sync intrinsics in that they specify a memory model and,
    more importantly for our purposes, they are reliably implemented
    either in the compiler or in libatomic.

    Fixes https://gcc.gnu.org/PR52084

    Reviewed-on: https://go-review.googlesource.com/c/160820

From-SVN: r268458