builtins: rely on the compiler for user label prefix

clang does not like the definition of builtins.  In order to work around this,
we use a SUN CC to redefine the generated name.  However, this requires that we
account for the user label prefix.  Rather than hard coding that into the file,
rely on the compiler to tell us the information and use the preprocessor to
generate the name as we do in the assembly routines.  NFC.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@224597 91177308-0d34-0410-b5e6-96231b3b80d8