builtins: repair the builtins build with clang 3.8

When the C unwinding personality was corrected to match the ARM EHABI
specification, the unwind header in clang was updated with necessary
declarations.  However, when building with an older compiler, we would not have
the necessary declarations.  This would result in a build failure.  Provide a
supplementary header to ensure that the necessary declarations are present for
the build of the C unwinding personality.

Note that this is NOT an ABI break.  It merely is a compile time failure due to
the constants not being present.  The constants here are reproduced
equivalently.  This header should permit building with clang[<3.9] as well as
gcc.

Addresses PR31035!

llvm-svn: 287359