[release-branch.r58] cgo: handle new Apple LLVM-based gcc from Xcode 4.2
    
    ««« CL 4607045 / 142f0bc0d6e7
    cgo: handle new Apple LLVM-based gcc from Xcode 4.2
    
    That gcc does not include enumerator names and values
    in its DWARF debug output.  Create a data block from which
    we can read the values instead.
    
    Fixes #1881.
    
    R=iant
    CC=golang-dev
    https://golang.org/cl/4607045
    »»»
    
    R=adg
    CC=golang-dev
    https://golang.org/cl/4708042