C API changed due to ARM64 backend renaming

Hi @ll,

currently we have a few defines in Target.h such as "#define LLVM_TARGET(TargetName) void LLVMInitialize##TargetName##Target(void);” that creates the list of initialization functions for all configured targets.

Because we renamed the the ARM64 backend to AArch64 we are no longer generating and exporting the old ARM64 initialization functions and existing clients won’t be able to link against the new library.

Could we add an alias init function for ARM64, which just calls the AArch64 init function, to not break existing clients?