GCC has had support for 64-bit ARM, a.k.a. AArch64, going back to last summer for using the open-source compiler with next-generation ARMv8 hardware. Being merged today is finally support for the LLVM compiler infrastructure with an experimental 64-bit ARM/AArch64 back-end target.
Adding fourty-five thousand lines of new code to the LLVM tree is a patch that adds support for ARM's 64-bit architecture to LLVM. This target is currently experimental and not built by default, but requires a compile-time switch for enabling. However, there's already talk amongst developers about enabling 64-bit ARM support by default. This work was merged into mainline LLVM this morning and will thus be found in the LLVM 3.3 release due out in a couple months time.