It's a GCC extension. https://gcc.gnu.org/onlinedocs/gcc/Labels-as-Values.html

Is there an equivalent for Clang/LLVM?

Yes.

http://blog.llvm.org/2010/01/address-of-label-and-indirect-b...

A venerable interpreter implementation trick.

Unfortunately it won't port to IAR, which is a shame. I don't understand why pointers to labels never made it to the C spec; it's one of the (equivaling) features that actually makes assembler look elegant.