[ARM,AArch64] Define __ELF__ for arm-none-eabihf and AArch64

This macro is defined for arm-none-eabi as of r266625, but it should also be
defined for eabihf and aarch64.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@300549 91177308-0d34-0410-b5e6-96231b3b80d8