[release-branch.r58] cgo: handle new Apple LLVM-based gcc from Xcode 4.2

[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