LLVM has had 64-bit ARM (a.k.a. AArch64) support going back several months now and present in LLVM 3.3. Since yesterday, LLVM officially supports the ARM Cortex-A57 processor. The ARM Cortex-A57 ARMv8 processor supports between one and four cores, supports native 64-bit ARM instructions with 32-bit legacy support, TrustZone, NEON, hardware virtualization, etc.
The LLVM ARM Cortex-A57 support landed in the mainline code-base with this code change by an ARM Holdings engineer. Also committed this week were other LLVM ARM improvements including Cortex-A53 improvements, AArch64 instruction improvements, etc.